Updated nan version.

Updated nan to using the most recent npm version
rather than a git url, as all important changes have
been merged, and this removes the need for git to
be installed to `npm install` couchnode.

Change-Id: I4a3718892f49b3368af46b40c8ab2cd6dd183f42
Reviewed-on: http://review.couchbase.org/33205
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>