Rails Upgrade and Refactor (please do not merge yet :smile:)

Remaining todo list before merging into master:

- [ ] Mock out API call to Yelp in test suite
- [ ] `/retailers` should have a response
- [ ] Bring back jquery app slide transitions
- [ ] Complete Readme updates
- [ ] Fix import endpoint for csv of addresses gone (but my day job fixes this! :smiley:)
- [ ] Verify that Ruby version in production supports Rails 4