chore(package): update yargs to version 15.0.2

Closes #91

Author: greenkeeper[bot]

demo/package.json

@@ -78,7 +78,7 @@
     "tslint": "~5.11.0",
     "typescript": "~3.2.2",
     "webpack-cli": "^3.2.1",
-    "yargs": "12.0.1"
+    "yargs": "15.0.2"
   },
   "jest": {
     "preset": "jest-preset-angular",