Allow ruby-head to fail for now
Allow ruby-head to fail for now

Rubygems has been broken since April 10th:

 * https://github.com/rubygems/rubygems/commit/620910aa75c7446e8f8d5af1df5f981e4da86197
 * https://github.com/rubygems/rubygems/issues/1220

yui-knk/did_you_mean,yuki24/did_you_mean,Ye-Yong-Chi/did_you_mean