Add rspec gem to list as well.

While it’s not normally needed (it has no code!),
some RSpec extensions depend directly on `rspec` and
in such situations you’ll need it to have the newer
version as well.

Author: myronmarston

README.md

@@ -14,7 +14,7 @@ Want to run against the `master` branch? You'll need to include the dependent
 RSpec repos as well. Add the following to your `Gemfile`:
 
 ```ruby
-%w[rspec-core rspec-expectations rspec-mocks rspec-support].each do |lib|
+%w[rspec rspec-core rspec-expectations rspec-mocks rspec-support].each do |lib|
   gem lib, :git => "git://github.com/rspec/#{lib}.git", :branch => 'master'
 end
 ```