Update build tools rev 21.0.2

Updated build tools to rev 21.0.2 in android build.gradle, building doc and travis.yml
This rev. solves the windows users issue:
https://code.google.com/p/android/issues/detail?id=77629

Travis wont work until they upgrade or you install 23.0.5 sdk manually,
emulator or something i dont know do. I ll wait your/their solution.
http://docs.travis-ci.com/user/languages/android/
https://github.com/travis-ci/travis-cookbooks/blob/master/ci_environment/android-sdk/CHANGELOG.md
Merge pull request #70 from ardock/update-build-tools-rev-21.0.2

Update build tools rev 21.0.2