fix(package): update yargs to version 13.2.4

Closes #32

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "png-file-stream": "^1.0.0",
     "puppeteer": "^1.2.0",
     "tempdir": "^2.0.0",
-    "yargs": "^12.0.1"
+    "yargs": "^13.2.4"
   },
   "devDependencies": {
     "mocha": "^5.2.0",