Fix specs with rails 4
No need for an Engine
Fix bundler + gemspec configuration

Using bundler + gemspec means `git ls-files` can't be used when testing new files.

Additionally, the spec files don't *require* bundler to run, so instead we can just `bundle exec rspec` to lock down the gemset then run as normal. Bundler makes sure our gem is already in the load path.
Merge pull request #8 from sj26/remove-engine

Remove engine