Ditch Git dependencies in favour of just mspec

This PR does two things:

1. Fix the Travis config so it _actually_ allows certain Rubies to fail
2. Remove the dependency on Git ffi/mspec and only use mspec from RubyGems

The latter fixes the annoying Bundler Git errors that would pop up all over the place. Having said that, are we ok with depending on a Gem version of mspec, or do we _want_ to use a Git version?