Make README.md instruct users to install 3.0.0

Author: Sam Phippen

README.md

@@ -12,7 +12,7 @@ Add `rspec-rails` to **both** the `:development` and `:test` groups in the
 
 ```ruby
 group :development, :test do
-  gem 'rspec-rails', '~> 3.0.0.beta'
+  gem 'rspec-rails', '~> 3.0.0'
 end
 ```