Upgrade Rails to 4.1.5.

Fixes security vulnerability with create_with. Read more here:
https://groups.google.com/forum/#!topic/rubyonrails-security/M4chq5Sb540

Note: Supermarket is not using create_with, so it is not vulnerable to this
issue. This upgrade is for extra precaution.
Merge pull request #749 from opscode/rails_4_1_5

Upgrade Rails to 4.1.5.