Adding Devise 3.1.x to Gemfile

The `gem "devise", "~> 3.1.0"` declaration indicates that I only want
to use versions of Devise that are in the 3.1.x family.
I believe Devise is using to [semantic versioning](http://semver.org/)

Devise is a "Flexible authentication solution for Rails with Warden."
https://github.com/plataformatec/devise

Warden is a "General Rack Authentication Framework"
https://github.com/hassox/warden

Rack is "a modular Ruby webserver interface"
https://github.com/rack/rack
http://rack.github.io/