Bump minimum Node version to 4.0

Author: Evan Jacobs

package.json

@@ -22,6 +22,9 @@
         "lodash": "^4.0.0",
         "mkdirp": "^0.5.1"
     },
+    "engines": {
+        "node": ">= 4"
+    },
     "scripts": {
         "postinstall": "which yo || npm i -g yo"
     }