fix: Bring back Node.js v6 support

It was removed (withuot major version bump) with v2.3.3

Author: medikoo

package.json

@@ -43,7 +43,7 @@
     "bluebird": "^3.7.2",
     "chalk": "^2.4.2",
     "fs-extra": "^8.1.0",
-    "googleapis": "^41.0.1",
+    "googleapis": "^39.2.0",
     "lodash": "^4.17.15"
   },
   "devDependencies": {