Android SDK: pre-install build-tools 21.1.1
Android SDK: pre-install build-tools 21.1.1

I removed former versions 20.0.0 (and transient 21.0.2).
As agreed in https://github.com/travis-ci/travis-cookbooks/pull/360,
we only pre-install the "latest" build-tools version

Distelli/travis-cookbooks,Zarthus/travis-cookbooks,Acidburn0zzz/travis-cookbooks,dracos/travis-cookbooks,Acidburn0zzz/travis-cookbooks,DanielG/travis-cookbooks,dracos/travis-cookbooks,tianon/travis-cookbooks,spurti-chopra/travis-cookbooks,dracos/travis-cookbooks,tianon/travis-cookbooks,vinaykaradia/travis-cookbook-cloned,ljharb/travis-cookbooks,dracos/travis-cookbooks,vinaykaradia/travis-cookbook-cloned,DanielG/travis-cookbooks,Zarthus/travis-cookbooks,travis-ci/travis-cookbooks,gavioto/travis-cookbooks,travis-ci/travis-cookbooks,spurti-chopra/travis-cookbooks,vinaykaradia/travis-cookbook-cloned,Zarthus/travis-cookbooks,ardock/travis-cookbooks,ardock/travis-cookbooks,Zarthus/travis-cookbooks,ljharb/travis-cookbooks,DanielG/travis-cookbooks,travis-ci/travis-cookbooks,Distelli/travis-cookbooks,vinaykaradia/travis-cookbook-cloned,gavioto/travis-cookbooks,Distelli/travis-cookbooks,tianon/travis-cookbooks,Acidburn0zzz/travis-cookbooks,spurti-chopra/travis-cookbooks,ardock/travis-cookbooks,DanielG/travis-cookbooks,tianon/travis-cookbooks,gavioto/travis-cookbooks,gavioto/travis-cookbooks,tianon/travis-cookbooks,vinaykaradia/travis-cookbook-cloned,Acidburn0zzz/travis-cookbooks,ardock/travis-cookbooks,ljharb/travis-cookbooks,Distelli/travis-cookbooks