Upgrade to rspec-rails 3.1

* Fix deprecation warnings in the cookie matcher
* Fix loading of the MiniTest runner (caused by requiring
  `rails/test_help`, which we didn't need anyway.
* Remove wrapper around MiniTest::Assertion
* Clean up requires in `spec_helper` and `env`.
* Remove .rspec file as the settings in there are default
* Add appraisals to version control per https://github.com/thoughtbot/appraisal#version-control