Upgrade to node 6.
Upgrade to node 6.

After Kartotherian and Tilerator have been switched to
node v6 in production, all of our services support running
under that node version. So, we can use it in Vagrant as well.

Change-Id: Ib97834a298e77c0c3940e87ff09c63b6ee1daf3c

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant