[FIX] cannot load such file -- minitest_helper (LoadError)

The [README](https://github.com/blowmage/minitest-rails-capybara#configure) instructs users to set up mini_test using the instructions given by [minitest-rails](https://github.com/blowmage/minitest-rails).

Following that procedure will create a file called `test_helper.rb`.  These commits will update the `minitest-rails-capybara` feature generators to `require "test_helper"` rather than `require "minitest_helper"`.

Without this fix, users who follow the README will need to do one of the following:

1.  `mv test/test_helper.rb test/minitest_helper.rb` or
2.  Edit each generated feature to require "test_helper" instead.