Upgrade NPM

This changeset upgrades the version of NPM from 1.4.28 to 2.1.14. Joyent bundles Node.js with the current version of NPM at time of release. In our case, 0.10.32.

Locally, this change appears to have increased the probability of build success. I'm going to do more testing, but putting this up here so that others can confirm.
I think this needs to be `unless Vagrant::Util::Platform.windows? || unless Vagrant::Util::Platform.cygwin?`