fix(package): update yargs to version 15.0.1

Closes #279

Author: greenkeeper[bot]

package.json

@@ -112,7 +112,7 @@
     "source-map-support": "^0.5.0",
     "strip-ansi": "^4.0.0",
     "toposort": "^1.0.0",
-    "yargs": "^11.0.0"
+    "yargs": "^15.0.1"
   },
   "greenkeeper": {
     "ignore": [