chore(ci): drop node 4 as a supported env (#478)

* chore(ci): drop node 4 as a supported env

* chore(pkg): update engine

.travis.yml

@@ -1,7 +1,6 @@
 language: node_js
 
 node_js:
-  - '4'
   - '6'
 
 dist: trusty

package.json

@@ -21,7 +21,7 @@
     "url": "https://github.com/mozilla/fxa-oauth-server/issues"
   },
   "engines": {
-    "node": ">=4.5.0"
+    "node": ">=6"
   },
   "dependencies": {
     "bluebird": "^2.9.14",