Install node + npm for mac, to go along with existing commands for linux.

Summary: Oops that I left this out before. :-(

Test Plan:
Ran the install_node() commands by hand on an OS X machine with both
node and npm already installed, and also tried running the 'brew' and
'curl' commands by hand even though they were already installed.

Reviewers: benkomalo, alpert

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2235