fix: drop support for nodejs 9.x (#161)

* fix: drop support for node.js 4.x and 9.x

* moar fixing

Author: JustinBeckwith

error-reporting/package.json

@@ -6,7 +6,7 @@
   "author": "Google Inc.",
   "repository": "googleapis/nodejs-error-reporting",
   "engines": {
-    "node": ">=4.3.2"
+    "node": ">=6.0.0"
   },
   "scripts": {
     "ava": "ava -T 20s --verbose test/*.test.js system-test/*.test.js",