Vendor `rails/test_help` and remove minitest

Removes minitest stuff from `rails/test_help` and vendors it.
Updates gemspec to allow versions of rails greater than 3.

Works for me™
Remove `test_help` entirely

Turns out for the Rails app's i've tried you don't need these lines at
all. Given we're relying on spinning up the app via rack-test.
restore fixture support
Merge pull request #256 from JonRowe/dumb_fix_for_rails4_test_help

(dumb) Fix for Rails4