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
Update mailer spec

* Remove `before` block
* Use I18n translations where available
* Separate setup, exercise, expectation
* Move to double quotes
* Add `raw` to email body to avoid weird character encoding
Move test app into spec/dummy

This isn't library code, it's test code. It doesn't belong in `lib`.
Common convention in engines is to have the dummy app in `spec/dummy`.

* Update namespace so it doesn't look like part of Clearance.
* Update reference from `Clearance::Testing::Application` to
  `Dummy::Application`.
* Move dummy rake tasks into the `dummy` rake namespace.
* Update bin/setup to use the new namespace.
* Update bin/setup to no longer clean old appraisals out.
* Update style in Rakefile and Dummy app
Update unit tests to reflect current style guide

* Remove use of `subject`and `let` statements
* Remove `before` blocks
* Move repetitive setup into private methods
* Move to double quotes