fix(deps): update dependency google-gax to ^0.20.0 (#193)

Author: renovate[bot]

packages/google-cloud-vision/package.json

@@ -66,7 +66,7 @@
   },
   "dependencies": {
     "@google-cloud/promisify": "^0.3.0",
-    "google-gax": "^0.19.0",
+    "google-gax": "^0.20.0",
     "is": "^3.2.1",
     "lodash.merge": "^4.6.1",
     "protobufjs": "^6.8.6",