fix(deps): update dependency @google-cloud/promisify to ^0.4.0 (#420)

Author: renovate[bot]

package.json

@@ -46,7 +46,7 @@
     "@google-cloud/common-grpc": "^0.10.0",
     "@google-cloud/paginator": "^0.1.0",
     "@google-cloud/projectify": "^0.3.0",
-    "@google-cloud/promisify": "^0.3.0",
+    "@google-cloud/promisify": "^0.4.0",
     "arrify": "^1.0.0",
     "concat-stream": "^2.0.0",
     "dot-prop": "^4.2.0",