drone: overcome grunt cli not global

.drone.yml

@@ -18,7 +18,7 @@ steps:
     image: node
     commands:
       - npm install
-      - grunt build:cli
+      - npm run build:cli
       - cd cli
       - npm install
       - npm test

package.json

@@ -25,6 +25,7 @@
     "node": ">=4.0.0"
   },
   "scripts": {
+    "build:cli": "grunt build:cli",
     "test": "npm run test:sunit && npm run test:e2e",
     "test:sunit": "grunt test",
     "test:e2e": "(cd cli && npm test)"