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.
* Reduce the overall Travis matrix size by not running a Travis job for each gemfile.

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.
* Add spring to development group for Rails 4.1
* Add therubyrhino for JRuby.
* Remove old references to Rails 3.
* Fix JRuby test where output is "1 runs", not "1 tests".
* Remove Rails 3.0 reference to "turn".