chore(package): update yargs to version 15.2.0

Closes #91

Author: greenkeeper[bot]

package.json

@@ -103,7 +103,7 @@
     "tsickle": "^0.33.0",
     "tslint": "~5.11.0",
     "typescript": "~3.2.4",
-    "yargs": "12.0.1",
+    "yargs": "15.2.0",
     "zone.js": "^0.8.26"
   },
   "engines": {