cleanup build cache according to travis docs

.travis.yml

@@ -10,8 +10,9 @@ jdk:
   - oraclejdk8
 
 before_cache:
-  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
- 
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+
 cache:
   directories:
     - $HOME/.gradle/caches/