Update yargs to version 15.0.2 (#298)

Closes #269

Co-authored-by: greenkeeper[bot] <23040076+greenkeeper[bot]@users.noreply.github.com>

Author: RyanZim

package.json

@@ -29,7 +29,7 @@
     "postcss-reporter": "^6.0.0",
     "pretty-hrtime": "^1.0.3",
     "read-cache": "^1.0.0",
-    "yargs": "^12.0.1"
+    "yargs": "^15.0.2"
   },
   "devDependencies": {
     "ava": "^1.0.1",