Update npm to v6

Author: renovate-bot

package.json

@@ -21,7 +21,7 @@
     "download": "https://github.com/Ferie/cssCollapse/archive/master.zip",
     "engines": {
         "node": "6.17.1",
-        "npm": "3.10.10"
+        "npm": "6.12.1"
     },
     "scripts": {
         "build": "gulp",