As travis-rubies now uses 3 Mac jobs to create archives for various OS
  releases.

  This is a bit wasteful if multiple builds pass in a short period.

  Instead, Travis CI is now running a nightly build of ruby-head
  (around 22:30 UTC). This will happen regardless of the state of the
  master branch.

  Author: Hiro Asari <asari.ruby@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56821 b2dd03c8-39d4-4d8f-98ff-823fe69b080e