fix(package): update yargs to version 15.0.1

Closes #24

Author: greenkeeper[bot]

package.json

@@ -57,7 +57,7 @@
     "uws": "^10.148.0",
     "vectorious": "^4.8.1",
     "write-file-atomic": "^2.3.0",
-    "yargs": "^11.0.0"
+    "yargs": "^15.0.1"
   },
   "devDependencies": {
     "babel-cli": "^6.26.0",