Update npm version and package-lock

README.md

 
 ## Building the sources
 
-Node.js >= 8 is required.
+Node.js >= 8 and npm >= 6 are required.
 
 On Debian/Ubuntu systems, the required packages can be installed with:
 ```

package.json

     "webpack-dev-server": "2.9.5"
   },
   "engines": {
-    "node": ">=6.0.0"
+    "node": ">=8.0.0",
+    "npm": ">=6.0.0"
   },
   "license": "Apache-2.0",
   "scripts": {