Added in ability for user to redefine update center plugin URL (thus releasing us from jenkins update center dependency)
Rework the support for custom update center URLs with jenkins::plugins to include tests

This change allows for using a custom update center URL with either versioned
plugins or unversioned plugins
Bind the Vagrant gem to specific platforms to try to make Travis happy
Removing rubies that cause all kinds of problems when testing
Exclude rubies where bundle install cannot seem to succeed due to bundler idiocy
Merge pull request #190 from jenkinsci/pr/134

Rework of custom update center usage