fix: drop support for node.js 4.x and 9.x (#73)

Author: JustinBeckwith

cloud-language/snippets/package.json

@@ -4,7 +4,7 @@
   "license": "Apache-2.0",
   "author": "Google Inc.",
   "engines": {
-    "node": ">=4.0.0"
+    "node": ">=6.0.0"
   },
   "repository": "googleapis/nodejs-language",
   "private": true,