fix(deps): update dependency google-gax to ^0.18.0 (#63)

Author: renovate[bot]

packages/google-cloud-bigquery-datatransfer/package-lock.json

@@ -46,7 +46,7 @@
     "system-test": "mocha system-test/*.js smoke-test/*.js --timeout 600000"
   },
   "dependencies": {
-    "google-gax": "^0.17.1",
+    "google-gax": "^0.18.0",
     "lodash.merge": "^4.6.1"
   },
   "devDependencies": {