Upping min node version

one of our modules uses the spread operator. That require node 8.3.0.

Author: makinde

.travis.yml

@@ -2,7 +2,7 @@ language: node_js
 services:
   - mongodb
 node_js:
-  - "8.2.1"
+  - "8.3.0"
 before_script:
   - sleep 15
 script:

package.json

@@ -2,7 +2,7 @@
   "name": "mongoose-authz",
   "version": "2.0.0-6",
   "engines": {
-    "node": ">=8.2.1"
+    "node": ">=8.3.0"
   },
   "description": "Data level authorization for Mongoose",
   "main": "index.js",