Make Travis builds work

- Moved development dependencies out to Gemfile so that we can have more
  fine-grained control over which ones get loaded.
- Make sure that 1.8.7 tests run by explicitly adding a development
  dependency on then 'json' gem (which isn't bundled with Ruby 1.8.7)
- Update the Travis configuration file to build against more modern
  Rubies and without development dependencies