fix(deps): update dependency yargs to v15 (#308)

Author: renovate-bot

cloud-tasks/package.json

@@ -16,7 +16,7 @@
     "@google-cloud/tasks": "^1.6.0",
     "body-parser": "^1.18.3",
     "express": "^4.16.3",
-    "yargs": "^14.0.0"
+    "yargs": "^15.0.0"
   },
   "devDependencies": {
     "chai": "^4.2.0",