Specify engine and drop support of node 4/5 🚨

.travis.yml

@@ -1,7 +1,5 @@
 language: node_js
 node_js:
-  - "4"
-  - "5"
   - "6"
   - "7"
   - "8"

package.json

@@ -3,6 +3,9 @@
   "version": "2.8.3",
   "description": "Datadog API Node.JS Client",
   "main": "lib/index.js",
+  "engines": {
+    "node": ">= 6.10.0"
+  },
   "scripts": {
     "test": "./node_modules/.bin/mocha --recursive ./test",
     "lint": "eslint lib test bin/dogapi examples",