use https for rubygems
remove activesupport dependency

- Hash#diff was removed from AS
- monkey patch hash if necessary to add Hash#diff
relax activemodel dependency
use pry-byebug instead of pry-debugger
update travis config

- ActiveModel => 4 requires ruby 1.9.3+
- add ruby 2.1.1
- cache bundled gems
update README
don't install debugging gems on travis
Merge pull request #5 from BM5k/rails-4.0

Rails 4.0