Fixes bench tests under JRuby

* It appears that require 'minitest/spec' pulls in the stdlib version of
  minitest, as opposed to the > 2.1 gemified version required by
  mini_shoulda. This causes conflicts and bizarre errors messages. Since
  mini_shoulda pulls in 'minitest/spec' as well (but the correct
  version), this line can simply be removed.

test/helper.rb

@@ -7,7 +7,6 @@
 require 'rails'
 puts "Testing with Rails #{Rails.version}"
 
-require 'minitest/spec'
 require 'mini_shoulda'
 require 'minitest/pride'
 require 'minitest/autorun'