Testing against ruby-head on Travis is a bit of a joke.
Travis CI has recommended adding all ruby-head builds to an allow_failures config:

https://github.com/travis-ci/travis-ci/issues/1195#issuecomment-20160541

I'd rather just leave it out of the config.
Merge pull request #30 from campaignmonitor/skip-travis-ruby-head

Testing against ruby-head on Travis is a bit of a joke.