(maint) Provide JUnit output for jenkins

This commit copies tasks/ci.rake and the ci:spec task from puppet
(5b723279b3ff14b1d411688f08608e669447c607). It also updates the Gemfile to
include the yarjuf gem in the developer/test sections.

Without JUnit style output we don't have a good way of seeing test
failures within jenkins. This adds a new rake task ci:spec that will
run with junit output, descriptive output and spec order logging.