Add caching (#155)

Summary:
Add some directories for caching. Also remove clean-up steps, which
aren't necessary.
Pull Request resolved: https://github.com/facebook/Sonar/pull/155

Reviewed By: jknoxville

Differential Revision: D8859224

Pulled By: passy

fbshipit-source-id: e8673504f5da74fbe5627c308ccd61fc28901116

.travis.yml

@@ -78,6 +78,13 @@ matrix:
           - android-sdk-preview-license-.+
           - android-sdk-license-.+
           - google-gdk-license-.+
+
+      cache:
+        directories:
+          - $HOME/.gradle/wrapper
+          - $HOME/.gradle/caches
+          - android/third-party/external
+
       before_install:
         - yes | sdkmanager "platforms;android-27"
         - echo "y" | sdkmanager "cmake;3.6.4111459"
@@ -91,7 +98,3 @@ matrix:
         - cd ../
       script:
         - ./gradlew :sample:build
-      after_success:
-        - rm -rf temp-android-ndk
-      after_failure:
-        - rm -rf temp-android-ndk