Update .travis.yml

Fixed gradle caching as per https://docs.travis-ci.com/user/languages/java/#projects-using-gradle

.travis.yml

@@ -26,10 +26,13 @@ jobs:
       - stage: "System Tests"
         name: "Test environment setup"
         script: bash scripts/prepareServerJars.sh
+before_cache:
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
 cache:
   directories:
     - '$HOME/.m2/repository'
     - '$HOME/.sonar/cache'
-    - '$HOME/.gradle'
-    - '.gradle'
+    - '$HOME/.gradle/caches/'
+    - '$HOME/.gradle/wrapper/'
     - '$HOME/servers/'