minitest-rails 0.9.2, shoulda-matchers 2.0.0, and deprecation updates

I've updated 2 outdated dependencies.
`minitest-rails` is now on `0.9.2` and `shoulda-matchers` in `2.0.0`.

Tests are all passing, but had to change a matcher (`respond_with_content_type`) because it was removed on `shoulda-matchers` 2.0.0. It's now using `respond_with` and test descriptions were updated.

A test for models was also added. I believe because of a side effect of `minitest-rails` 0.9.2 some matchers now need to be more explicit.
`ActiveSupport::TestCase` now includes `MiniTest::Spec::DSL`, and instead of inheriting from `MiniTest::Spec` it inherits from `MiniTest::Unit::TestCase`
`minitest-matchers` defines `self.must`, etc on `MiniTest::Spec`, and I believe there's the problem.

I'll try to keep investigating, but everything else seems to be working OK!

Tested it on Ruby 1.9, and Ruby 2.0 with Rails 4.