Improve setup for contributors

I split this work into four commits. Hopefully this will get Travis passing on
master for the first time in a year!

Add bin/setup, improve contributing guidelines
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Move manual Appraisal tasks into bin/setup.

Update Travis to run against latest Ruby, Rails
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* Set up Appraisal before running `rake`.
* Add Ruby 2.1.2 to the testing matrix.

Ignore gemfiles, don't ignore Gemfile.lock in git
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Move dev dependencies out of gemspec and into Gemfile to gain more control
over the private API.

Test against Rails 3.2, 4.0, 4.1
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* 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.