Enable build cache for gradle.

This should reduce build time.

.travis.yml

@@ -5,7 +5,15 @@ before_install:
 - openssl aes-256-cbc -K $encrypted_eba2f7543984_key -iv $encrypted_eba2f7543984_iv
   -in travis/travisci_builder_id_key.enc -out travis/travisci_builder_id_key -d
 - chmod 600 travis/travisci_builder_id_key
-cache: ccache
+before_cache:
+  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+cache:
+  ccache: true
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/
+    - $HOME/.android/build-cache
 install: travis/install_extra_deps.sh
 script: travis_wait 30 travis/compile_all.sh
 deploy: