fix: remove Node.js 4 support

Lambda has deprecated the Node.js 4 runtime and is no longer accepting updates. Travis was failing one Node.js 4 due to the dependency updates.

Author: brettstack

.travis.yml

@@ -1,7 +1,6 @@
 language: node_js
 
 node_js:
-  - 4
   - 6
   - 8
   # - node # runs tests against latest version of Node.js for future-proofing

package.json

@@ -26,7 +26,7 @@
     "url": "https://github.com/awslabs/aws-serverless-express.git"
   },
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "release": {
     "verifyConditions": [