chore: require node 8 for samples (#61)

Author: JustinBeckwith

packages/google-cloud-bigquery-datatransfer/samples/package.json

@@ -4,7 +4,7 @@
   "license": "Apache-2.0",
   "author": "Google Inc.",
   "engines": {
-    "node": ">=6.0.0"
+    "node": ">=8"
   },
   "repository": "googleapis/nodejs-bigquery-data-transfer",
   "private": true,