fix(deps): update dependency google-gax to ^0.20.0 (#302)

Author: renovate[bot]

handwritten/bigtable/package.json

@@ -67,7 +67,7 @@
     "escape-string-regexp": "^1.0.5",
     "extend": "^3.0.0",
     "google-auth-library": "^2.0.0",
-    "google-gax": "^0.19.0",
+    "google-gax": "^0.20.0",
     "is": "^3.0.1",
     "is-utf8": "^0.2.1",
     "lodash.flatten": "^4.2.0",