Are you using Heroku?
Heroku will inject plugins in Rails 3.x applications ..
To avoid this
injection in Rails 3, include the rails_12factor gem in your
application. (Heroku Ruby Support 2013-10-26)
The rails_12factor gem is also required in rails 4.
If this gem is not present in your application, you will receive a
warning while deploying, and your assets and logs will not be
functional. (Rails 4 on Heroku 2013-10-26)
As recently as 2013-08, heroku always injected plugins in rails 3 apps, even apps with the recommended gems. This was an issue with the ruby buildpack, and was fixed by PR 11, merged on 2013-08-06.