Improved Travis-CI config

- Use default JDK
- Cache management for faster builds

.travis.yml

@@ -1,4 +1,8 @@
-language: groovy
-
-jdk:
-  - oraclejdk8
+language: java
+before_cache:
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/