Rails 4.2 support, part 1

1. Move the Rails-stable gemfile to Rails-4.2, and update to use the Rails 4.2 dependencies.  As a side note, the Rails-stable gemfile was actually just testing 4.1
2. Support Symbol (Rails 4.0/4.1) or String (Rails 4.2) keys when looking up reflection
3. Split up a method in lib/history.rb for easier debugging
4. Add some test-specific detail to an error message for a spec failing under Rails 4.2
5. Update Travis CI config to account for these changes.

There's still one failing spec on Rails 4.2, as well as a number of deprecation warnings.  More to come.