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

apache/calcite-avatica,looker-open-source/calcite-avatica,apache/calcite-avatica,apache/calcite-avatica,apache/calcite-avatica,looker-open-source/calcite-avatica,looker-open-source/calcite-avatica,looker-open-source/calcite-avatica,looker-open-source/calcite-avatica,apache/calcite-avatica