Run load hooks after setting up spec DSL

v0.9 slightly broke Draper's integration because we had `require 'minitest/rails'` in order to make sure the Rails `TestCase`s had the spec DSL (we need to `register_spec_type` for decorators). Now that require statement also pulls in `minitest/autorun`, which causes our class to be run, albeit with zero tests.

For other gems to be able to call `register_spec_type`, they need to be able to sneak in after Minitest::Rails works its magic. I think the simplest approach is to provide a load hook.