maint - Add temp workaround for travis ruby 1.8.7

Rubygems broke travis builds on ruby 1.8.7:

travis-ci/travis-ci#1793
rubygems/rubygems#763

This patch is a temporary workaround until rubygems is updated.

Copied from: https://github.com/puppetlabs/puppet/commit/d58c7f04
(MCO-20) add unit tests for MCollective::Facts::Yaml_facts

This commit adds unit tests for the yaml_facts facts plugin.