Remove files that change often from Travis cache, remove broken files automatically

The cache itself is useful, however, there might be two problems:
* Corrupted files might get cached, so the subsequent builds would fail
* Too large cache might slowdown the build

cleanup-gradle-cache solves both issues.

The cache itself is still re-uploaded on every build (see "store build cache"),
so it might need more investigations.
diff --git a/.travis.yml b/.travis.yml
index d9198e1..0440e9b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -44,10 +44,8 @@
   depth: 10000
 #sudo: required
 before_cache:
-  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
-  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+  - F=CleanupGradleCache sh -x -c 'curl -O https://raw.githubusercontent.com/vlsi/cleanup-gradle-cache/v1.x/$F.java && javac -J-Xmx128m $F.java && java -Xmx128m $F'
 cache:
   directories:
     - $HOME/.gradle/caches/
     - $HOME/.gradle/wrapper/
-# End .travis.yml