Update dependency @google-cloud/datastore to v1.4.2 (#863)

Author: renovate[bot]

functions/datastore/package.json

@@ -19,7 +19,7 @@
     "test": "npm run system-test"
   },
   "dependencies": {
-    "@google-cloud/datastore": "1.3.4",
+    "@google-cloud/datastore": "1.4.2",
     "supertest": "^3.0.0"
   },
   "devDependencies": {