Bump minitest-rails development dependency

Draper should depend on the released versions of minitest-rails.
Version ~> 1.0 for Rails 3.0-4.0, and ~> 2.0 for Rails 4.1+.
Remove minitest-rails require

The mintest/rails require should only be in the test helper.
This code is here to deal with very old versions of minitest-rails,
but Draper should rely on the 1.0+ released versions.
These checks are very outdated, and worse, can introduce other bugs.
Use load hook for minitest integration

Unlike RSpec, Minitest doesn't load the Spec DSL when loaded by Bundler,
so loading the integration in the Railtie can be problematic.
Switch to a load hook so that the integration can be loaded whenever
minitest-rails is loaded, which it may not be during Draper initialization.
Merge pull request #622 from blowmage/minitest-rails-release

Update minitest-rails integration