Test against Rails 3.2, 4.0, 4.1

* Remove gemfiles from version control to match same testing style as
  Clearance and Suspenders.
* MiniTest does not need to be manually included as it is automatically
  included by Rails by default.
Ignore gemfiles, don't ignore Gemfile.lock in git

Move dev dependencies out of gemspec and into Gemfile to gain more
control over the private API.
Update Travis to run against latest Ruby, Rails

* Set up Appraisal before running `rake`.
* Add Ruby 2.1.2 to the testing matrix.
Add bin/setup, improve contributing guidelines

Move manual Appraisal tasks into bin/setup.