chore(package): Require npm 3.10.8 or later

Update the `engines` field to include only the earliest version of the
current LTS line of releases and the version of npm included with that
release.

package.json

@@ -34,6 +34,7 @@
     "eslint": "^4.12.1"
   },
   "engines": {
-    "node": ">=6"
+    "node": ">=6.9.0",
+    "npm": ">=3.10.8"
   }
 }