Bump required Ruby version to 2.0.0

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

I think that, before the exact minimum version is fully decided, it is clear
that **at least** version 2.0.0 **must** be required by the `gemspec` files.

@rafaelfranca is that a correct assumption?
should be `Rails 5 prefers to run on Ruby 2.1 or newer.` for now.