Loosens Rails version dependency

Locking to ~> 3.2.6 makes experimental non-Rails 4 compatible.
As far as I know, everything should work fine on Rails 3.1+ (and
possibly as far back as 3.0.x). Loosening to >= 3.1.0 to fix this issue.

Bumps version
Sets Rails dependency to <= 5.0
Merge pull request #14 from howaboutwe/loosen_rails_dependency

Loosens Rails version dependency