Update AppInfo.cs
Update AppInfo.cs

In the previous version there is not "using System.Collections.Generic;" useful to use Dictionary.
At JsonHelper.Serialize(keys) the variable "keys" is not defined, so I change directly the value passed as jsonString.

This work on WP 8.1
danmichaelo/cordova-plugin-appinfo,danmichaelo/cordova-plugin-appinfo,danmichaelo/cordova-plugin-appinfo