Update docs to note that Gradle 3 is not supported

Breaking API changes in Gradle 3.0 make it impossible to support
it reliably alongside Gradle 1 and 2 without mainintaining multiple
versions of our Gradle plugin. This commit updates the documentation
to note that Gradle 3 is not supported.

Closes gh-6880

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -454,7 +454,7 @@ BootRepackage task and instructed it to work with only `clientJar` task and
 ----
 
 The configuration that we are referring to in `BootRepackage` is a normal
-http://www.gradle.org/docs/current/dsl/org.gradle.api.artifacts.Configuration.html[Gradle
+{gradle-dsl}/org.gradle.api.artifacts.Configuration.html[Gradle
 configuration]. In the above example we created a new configuration named
 `mycustomconfiguration` instructing it to derive from a `runtime` and exclude the `log4j`
 group. If the `clientBoot` task is executed, the repackaged boot jar will have all
@@ -598,8 +598,7 @@ using Gradle and how you're trying to publish the artifacts.
 ==== Configuring Gradle to produce a pom that inherits dependency management
 The following is an example of configuring Gradle to generate a pom that inherits
 from `spring-boot-starter-parent`. Please refer to the
-http://www.gradle.org/docs/current/userguide/userguide.html[Gradle User Guide] for
-further information.
+{gradle-user-guide}/userguide.html[Gradle User Guide] for further information.
 
 [source,groovy,indent=0,subs="verbatim,attributes"]
 ----

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -37,12 +37,13 @@ diverge from the defaults.
 [[getting-started-system-requirements]]
 == System Requirements
 By default, Spring Boot {spring-boot-version} requires http://www.java.com[Java 7] and
-Spring Framework {spring-version} or above. You can use Spring Boot with Java 6 with some additional
-configuration. See <<howto.adoc#howto-use-java-6>> for more details. Explicit build support
-is provided for Maven (3.2+) and Gradle (1.12+).
+Spring Framework {spring-version} or above. You can use Spring Boot with Java 6 with some
+additional configuration. See <<howto.adoc#howto-use-java-6>> for more details. Explicit
+build support is provided for Maven (3.2+) and Gradle (1.12 or 2.x). Gradle 3 is not
+supported.
 
-TIP: Although you can use Spring Boot with Java 6 or 7, we generally recommend Java 8 if at
-all possible.
+TIP: Although you can use Spring Boot with Java 6 or 7, we generally recommend Java 8 if
+at all possible.
 
 === Servlet containers
 The following embedded servlet containers are supported out of the box:
@@ -200,8 +201,9 @@ scope.
 
 [[getting-started-gradle-installation]]
 ==== Gradle installation
-Spring Boot is compatible with Gradle 1.12 or above. If you don't already have Gradle
-installed you can follow the instructions at http://www.gradle.org/.
+Spring Boot is compatible with Gradle 1.12 or 2.x. 2.14.1 is recommended. Gradle 3 is not
+supported. If you don't already have Gradle installed you can follow the instructions at
+http://www.gradle.org/.
 
 Spring Boot dependencies can be declared using the `org.springframework.boot` `group`.
 Typically your project will declare dependencies to one or more
@@ -213,8 +215,7 @@ that can be used to simplify dependency declarations and to create executable ja
 ****
 The Gradle Wrapper provides a nice way of "`obtaining`" Gradle when you need to build a
 project. It's a small script and library that you commit alongside your code to bootstrap
-the build process. See http://www.gradle.org/docs/current/userguide/gradle_wrapper.html
-for details.
+the build process. See {gradle-user-guide}/gradle_wrapper.html for details.
 ****
 
 Here is a typical `build.gradle` file:

spring-boot-docs/src/main/asciidoc/index.adoc

@@ -42,6 +42,8 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :gradle-userguide: http://www.gradle.org/docs/current/userguide
 :propdeps-plugin: https://github.com/spring-projects/gradle-plugins/tree/master/propdeps-plugin
 :ant-manual: http://ant.apache.org/manual
+:gradle-user-guide: https://docs.gradle.org/2.14.1/userguide
+:gradle-dsl: https://docs.gradle.org/2.14.1/dsl
 // ======================================================================================
 
 include::documentation-overview.adoc[]

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -791,7 +791,7 @@ If you can't directly import your project into your IDE, you may be able to gene
 metadata using a build plugin. Maven includes plugins for
 http://maven.apache.org/plugins/maven-eclipse-plugin/[Eclipse] and
 http://maven.apache.org/plugins/maven-idea-plugin/[IDEA]; Gradle offers plugins
-for http://www.gradle.org/docs/current/userguide/ide_support.html[various IDEs].
+for {gradle-user-guide}/userguide.html[various IDEs].
 
 TIP: If you accidentally run a web application twice you will see a "`Port already in
 use`" error. STS users can use the `Relaunch` button rather than `Run` to ensure that