Show Android SDK Tools Version

References:

 - https://github.com/travis-ci/docs-travis-ci-com/commit/d6d543acec8b2dcaa07e4a87012e1f01cb82d6a2
 - https://github.com/travis-ci/travis-ci/issues/2848#issuecomment-64181648
 - https://github.com/travis-ci/travis-ci/issues/2931
 - https://github.com/travis-ci/travis-cookbooks/commit/c7c0f1b4921a31e115af8573741bf9aaf4f07d59
this is only a single line output :)

Example:

```
vagrant@precise64:/usr/local/system_info$ grep Pkg.Revision /usr/local/android-sdk/tools/source.properties | awk -F= '{ print $2 }'
23.0.5
```