Add a per system run autoheal button

Added a button on Subscriptions Pane of Systems Page to attach
subscriptions to a single system.
Changes to make katello run on Ruby 2.0
Merge pull request #2671 from pgodhani/heal_single_system

Add a per system run autoheal button
Merge pull request #2676 from parthaa/2.0-changes

Changes to make katello run on Ruby 2.0