Update `npm test` to use the local mocha install.

Change-Id: Ie4814e192249d45755cc792b91c2fa9d9760fc4d
Reviewed-on: http://review.couchbase.org/30180
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>