Travis builds are failing

[barbeau]

We're now seeing this on all Travis builds:

* What went wrong:
Execution failed for task ':opentripplanner-android:preBuild'.
failed to find Build Tools revision 19.1.0

Example of failing build:
https://travis-ci.org/CUTR-at-USF/SiriRestClientUI/builds/28784544

It appears that the Android SDK license has changed with the release of L preview:
https://twitter.com/travisci/status/482255297738985473

Here's an example of the new license to include:
https://gist.github.com/BanzaiMan/545cf6d6fe9639c7c7cd#file-travis-yml-L9

Here's the Travis issue describing the problem:
travis-ci/travis-ci#2466