fix warning about loading minitest/unit

When loading minitest/unit in a Rails 4.1.0.rc1 app in ruby 2.1 with rspec-rails 3.0.0.beta2, the following warning occurs:

Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'

This patch appears to fix it.