Adds the What-Would-Travis-Do gem to run the full set of tests locally.

[wwtd](https://rubygems.org/gems/wwtd) uses the `.travis.yml` file to run the set of tests locally.

I haven't been able to run `rake test` without a bunch of `MiniTest::Unit::TestCase is now Minitest::Test` errors; this should be a convenient way to work around that problem without needing to run `BUNDLE_GEMFILE=gemfiles/Gemfile.rails-edge bundle exec rake test` or similar.