Update README.md 

When deploying a Rails 4.1 app it would fail on any `rake` task, unless I used the `bundle exec` command. 

This meant that Pushion Passenger tried to start the server it gave me the error: `uninitialized constant ActionDispatch::XmlParamsParser`

Adding the required fixed the issue.
Merge pull request #5 from richrace/update-docs

Update README.md [ci skip]