fix(deps): update dependency google-gax to ^0.18.0 (#278)

Author: renovate[bot]

handwritten/spanner/package-lock.json

@@ -71,7 +71,7 @@
     "events-intercept": "^2.0.0",
     "extend": "^3.0.1",
     "google-auth-library": "^1.6.1",
-    "google-gax": "^0.17.1",
+    "google-gax": "^0.18.0",
     "google-proto-files": "^0.16.1",
     "is": "^3.2.1",
     "lodash.merge": "^4.6.1",