Fix Travis-CI build

This is another attempt at what was started in #232.

We already use Appraisal for dependency regression tests. The .travis.yml file had a gemfiles declaration that competes with Appraisal. Combining the two caused Travis to blow up because Travis tries to run the suite against a particular (generated) gemfile, but the default rake task cleans that same gem file.

I'll pull this in once the Travis build goes green for this PR.