Specify android build-tools in .travis.yml

From July 2014 it is necessary to specify which build-tools version should
be used when building an Android project with Travis CI.

http://docs.travis-ci.com/user/languages/android/
Merge pull request #1090 from JessThrysoee/android-build-tools

Specify android build-tools in .travis.yml
Silence the JavacFiler warning 'compiler.warn.proc.unclosed.type.files'

This prevents warnings like:

   warning: Unclosed files for the types '[dummy1407426008266]'; these types will not undergo annotation processing

Files created via the createResource method are not registered for annotation
processing (as opposed to files created by the createSourceFile method), so we
avoid the JavacFiler warning and we don't have to open/close a stream.