Revise rake tasks

Make the rake tasks more closely match Rails 4.0.
The `rake minitest` task will now run the default tests in one task.
The `rake minitest:all` task will now run all tests in one task.
The `rake minitest:all:quick` task will now run all tests in one task
without resetting the db, making it just a little bit faster.
Support Minitest 5's API

Add minitest-test as a dependency, which allows Minitest::Test.
This will help folks who are confused by the Minitest's API change.