Re-write the tests for Railtie in Rspec or the rest of the tests in Minitest

[yuki24]

I had to use Minitest to move quickly and write tests for Artemis::Railtie, but obviously it is very odd to have two different testing frameworks in one gem. Here are some considerations:

• Rspec does not have out-of-box support for Railtie testing and would require a significant amount of exploration and additional code
• Rspec does not have a parallel test runner, which makes the test suite very slow (each Railtie test runs rails new to fully replicate a Rails app)
• MInitest does not have a nice command that runs a single spec (Rspec is very good at this and Minitest is way behind)
• The expect syntax generates an error message that's a lot more helpful than a simple assert