build(deps): bump yargs from 14.2.2 to 15.0.1 (#153)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ANSIREGEX-1583908

Author: snyk-bot

Diff for: package-lock.json

@@ -68,7 +68,7 @@
     "serve-static": "^1.12.1",
     "update-notifier": "^4.1.0",
     "yargonaut": "^1.1.2",
-    "yargs": "^14.2.0"
+    "yargs": "^15.0.1"
   },
   "devDependencies": {
     "ava": "^3.13.0",