check if the version of nodejs is already installed, and if so do nothing

This adds a step that checks if the requested version of nodejs is already installed. If it is, we skip the remaining steps. Without this, every time I provision it recompiles and installs nodejs.
The "changed_when" parameter makes it so that this check doesn't cause the playbook to reflect a change, but it wasn't introduced until Ansible 1.3 so I updated that in the meta config.