Update documentation for using GitHub source of gem, fixes ianfleeton#13

Author: ianfleeton

README.rdoc

@@ -15,7 +15,7 @@ You can use this fork which is actively maintained to ensure it is working prope
 == Installation
 
 Add to your Gemfile
-  gem 'paypal-express', github: 'ianfleeton/paypal-express'
+  gem 'ianfleeton-paypal-express', github: 'ianfleeton/paypal-express', require: 'paypal/express'
 Or
   gem 'ianfleeton-paypal-express', require: 'paypal/express'