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`.
rspec-rails read me says:

[NOTE: Generators run in RSpec 3.x will now require rails_helper instead of spec_helper.](https://github.com/rspec/rspec-rails#upgrade-note)

I was using the latest gem 'rspec-rails', '~> 3.0.2' and without rails_helper, I was getting Uninitialized SomeModule::ClassName error.