Add support for Rails 5.0

[Alexey Mikhaylov]

Simply adding Rails 5.0 to dependency leads to broken tests.

[Alex Pletnov]

Are there any plans on Rails 5 support?

[Thorben Starsy]

Hey @epaminond et al, there is support for Rails 5 now through the rails-5 and gem versions >= 1.0.0 on rubygems.org

I've also put in a PR to update the Readme.