fix(deps): update dependency @google-cloud/promisify to ^0.4.0 (#609)

Author: renovate[bot]

package.json

@@ -51,7 +51,7 @@
   "dependencies": {
     "@google-cloud/common": "^0.31.0",
     "@google-cloud/paginator": "^0.1.0",
-    "@google-cloud/promisify": "^0.3.0",
+    "@google-cloud/promisify": "^0.4.0",
     "arrify": "^1.0.0",
     "async": "^2.0.1",
     "compressible": "^2.0.12",