Warn if no 'build-tools' is specified in an Android project

As discussed in https://github.com/travis-ci/travis-cookbooks/pull/360#issuecomment-52902921
Communicate with the user that build-tools should be specified.

If there is a possibility that an Android project does not need one,
then we should detect it and suppress this warning.
Local tests looked good, I'll have to test this on staging, then.