zapnap/resque_mailer — pull request #57 by Empact (2013-05-05 09:05:24+00:00)
Title: Update to actionmailer dependency to >= 3.0, for compatibility with rails 4 pre-release.

Other gems, e.g. factory_girl, rspec-rails, use this dependency setup, presumably for the same reason.

Bundler will default to using the latest non-release version of the dependency, but will accomodate the pre-release version if it's required directly or by another dependency.