added ansible task to add chris-lea ppas for current version nodejs

I added a task to use chris-lea's repos for nodejs.  In other parts of the program, it seems like these repos are preferred to the default ubuntu repos ([see this](https://github.com/edx/edx-platform/blob/master/requirements/system/ubuntu/apt-repos.txt)).  This will keep the nodejs version current as well.

Separately, I fixed a typo where "npm" was listed twice.  Although "nodejs" does not need to be explicitly listed because it is a dependency of npm, I added it for people that may not be familiar with npm.
ok, I can change all instances of -npm to -nodejs.

I used [this search](https://github.com/edx/configuration/search?q=npm&ref=cmdform) to determine that I'll probably need to change the following files (in addition to the changes already made):

.travis.yml
playbooks/roles/jenkins_admin/defaults/main.yml 
playbooks/roles/jenkins_admin/tasks/main.yml 
playbooks/roles/xqueue/defaults/main.yml 
playbooks/roles/xqueue/tasks/main.yml 
playbooks/roles/jenkins_worker/defaults/main.yml 
playbooks/roles/jenkins_worker/tasks/system.yml 

@hkim823 Do you see any others that would need to be changed?  Also, the jenkins_admin task uses the ansible "with_items" approach as opposed to the "{{','.join(debian_pkgs)}}", while I'm at it should I change that to make it consistent with the others or just leave it?