drop openjdk7 build. sbt 1.x does not work with Java7

Author: xuwei-k

.travis.yml

@@ -8,18 +8,5 @@ scala:
 env:
 - JDK=oraclejdk8
 - JDK=openjdk8
-- JDK=openjdk7
-addons:
-  # https://github.com/travis-ci/travis-ci/issues/5227#issuecomment-165131913
-  hosts:
-    - scala-logging
-  hostname: scala-logging
 before_script:
   - jdk_switcher use $JDK
-matrix:
-  # scala 2.12 and 2.13 require java 8
-  exclude:
-    - scala: 2.12.4
-      env: JDK=openjdk7
-    - scala: 2.13.0-M2
-      env: JDK=openjdk7