fix(package): update yargs to version 15.0.2

Closes #232

Author: greenkeeper[bot]

package.json

@@ -86,7 +86,7 @@
     "webpack-merge": "4.2.2",
     "which": "1.3.1",
     "workbox-webpack-plugin": "3.6.3",
-    "yargs": "13.2.2"
+    "yargs": "15.0.2"
   },
   "devDependencies": {
     "@babel/cli": "7.6.2",