Updated rspec testing for Rails in README

I updated the README to remind users that since RSpec 3.x, rspec-rails is using `rails_helper` instead of  `spec_helper`.
Ok, I want this section to be shorter. We don't need to document all the little details of integration with Rails or a specific version of RSpec. Why not just remove the whole `require 'spec_helper'` altogether, it doesn't really pertain to the Grape testing part?