Update README installation instructions for Rails 3.x

I've updated the README because the original one was missing some steps. In order for the gem to work in Rails 3.x, you need to use the `git://` version of the URL in the Gemfile, not `git@github`. And in `config/application.rb`, you must `require rack/redis_throttle`. Otherwise, you will get `uninitialized constant Rack::RedisThrottle (NameError)`.