'npm test' command broke

FAB-2434
the way to launch headless tests has changed but the "test"
script for npm was left behind.

Change-Id: Ie29acb05a87222dd81175dd9dc97c77f9e3665b7
Signed-off-by: Jim Zhang <jzhang@us.ibm.com>

package.json

@@ -7,7 +7,7 @@
     "url": "http://gerrit.hyperledger.org/r/fabric-sdk-node"
   },
   "scripts": {
-    "test": "node test/unit/headless-tests.js"
+    "test": "gulp test-headless"
   },
   "engines": {
     "node": ">=4.5.0",