Plugin init compatible with Rails 4

The plugin initialization checks for Rails version 2 or 3, but nothing is done for Rails 4.

I don't know if there is a better way to check for Rails versions, but looking for `3` or `4` is the simplest working change.

Feel free to improve ;)

PS : I've filed a ticket on NewRelic support site about this, but having an issue tracker within the GitHub project would be very useful.