From https://docs.travis-ci.com/user/caching:
On Ruby and Objective-C projects, installing dependencies via Bundler
can make up a large portion of the build duration. Caching the bundle
between builds drastically reduces the time a build takes to run.
The Ruby 1.8.7 tests have been broken for ages, and it's unlikely anyone
will do the work to restore support for the tests. Drop them from the suite
to save Travis CI from doing work for nothing on every commit.
* Add ruby 2.1.0
* Add back ruby 1.8.7 as allowed failure - shows test results
but won't mark build as failed
* Enable fast finish - quick feedback on failed builds
* Make test matrix explicit so it's easier to tell what's included
in the matrix, instead of excluding certain combinations