JAVA-2560 : Update contributing guide

Specify that Java 9 should be used to compile and run the tests.

Author: jyemin

CONTRIBUTING.md

@@ -17,12 +17,12 @@ Pull Requests
 
 Pull requests should generally be made against the master (default) branch and include relevant tests, if applicable. 
 
-Code should compile with a Java 9 or later `javac` executable and tests should pass under all Java versions which the driver currently
+Code should compile with the Java 9 compiler and tests should pass under all Java versions which the driver currently
 supports. Currently the Java driver supports a minimum version of Java 6.  Please run './gradlew test' to confirm.   By default, running the
 tests requires that you start a mongod server on localhost, listening on the default port and configured to run with
 [`enableTestCommands`](http://docs.mongodb.org/manual/reference/parameters/#param.enableTestCommands), which may be set with the 
 `--setParameter enableTestCommands=1` command-line parameter.   At minimum, please test against the latest release version of the MongoDB 
-server and the latest version of Java currently available.
+server.
 
 The results of pull request testing will be appended to the request. If any tests do not pass, or relevant tests are not included, the 
 pull request will not be considered.