chore(package): update yargs to version 15.2.0

Closes #30

Author: greenkeeper[bot]

demo/package.json

@@ -63,7 +63,7 @@
     "tslint": "~5.9.1",
     "typescript": "~3.2.4",
     "webpack-cli": "^3.3.2",
-    "yargs": "13.2.2"
+    "yargs": "15.2.0"
   },
   "jest": {
     "preset": "jest-preset-angular",