chore: upgrade node >= 7.0.0

Need upgrade to pass linter errors:
The 'Object.getOwnPropertyDescriptors' is not supported until
Node.js 7.0.0. The configured version range is '>= 6.4.0'
node/no-unsupported-features/es-builtins

package.json

@@ -73,7 +73,7 @@
     "zuul": "^3.12.0"
   },
   "engines": {
-    "node": ">= 6.4.0"
+    "node": ">= 7.0.0"
   },
   "homepage": "https://github.com/visionmedia/superagent",
   "husky": {