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.
It returns what's in STDOUT after the command is executed in a sub shell. http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-60

It'll be bad if there is something in STDERR. We could add `2>/dev/null` or some such.