Version 2.0.0 / Rails 4+ changes

1. Updated dependencies so that we support Rails 4+ only.
2. Bumped version to 2.0.0
3. Neatened up Gemfiles so they only contain required lines and delegate to the gemspec.
4. Fixed use of 'http://rubygems.org' in Gemfiles
5. Updated CHANGELOG and moved to Markdown.
6. Fixed deprecation warnings in specs.
7. Removed Rails 3.x gemfiles and Travis entries.

@elight Can you review these changes?  Once they're merged to master it probably makes sense to push a gem to Rubygems, unless there's anything else you think should go in 2.0.0