Restrict FriendlyID to 4.x branch

For Rails3 compatibilty it's best to use the 4.x stable branch of
FriendlyId. FriendlyId5 is only compatible with Rails4+.

https://github.com/norman/friendly_id/blob/master/README.md
Merge pull request #4 from 18F/feature/restrict_friendlyid_to_4.0

Restrict FriendlyID to 4.x branch