Merge pull request #597 from mozilla/greenkeeper-yargs-4.6.0

Update yargs to version 4.6.0 🚀

Author: tofumatt

package.json

@@ -72,7 +72,7 @@
     "sha.js": "2.4.5",
     "source-map-support": "0.4.0",
     "xmldom": "0.1.22",
-    "yargs": "4.4.0",
+    "yargs": "4.6.0",
     "yauzl": "2.4.1"
   }
 }