MDL-60503 npm: Lock required node 8.9.x (LTS)

.travis.yml

@@ -55,7 +55,7 @@ matrix:
     include:
           # Run grunt/npm install on highest version ('node' is an alias for the latest node.js version.)
         - php: 7.1
-          env: DB=none     TASK=GRUNT   NVM_VERSION='node'
+          env: DB=none     TASK=GRUNT   NVM_VERSION='8.9'
 
     exclude:
         # MySQL - it's just too slow.

package.json

@@ -20,6 +20,6 @@
     "xpath": "0.0.23"
   },
   "engines": {
-    "node": ">=4"
+    "node": "8.9"
   }
 }