Speed up Travis builds by only running affected tests

[zombiezen]

Bazel has a script that already does this: https://github.com/bazelbuild/bazel/blob/master/scripts/ci/ci.sh

This would just need to be grafted onto the Travis configuration to use TRAVIS_COMMIT_RANGE, as described in Travis Environment Variables.

[bassdeveloper]

Hi @zombiezen , can I take this up?

[zombiezen]

Sure! Go for it!