Fixed Plugin for 3.0.2

As you probably know, Sketch will switch its scripting backend from JSTalk to CocoaScript in the upcoming version 3.0.2.

As a result of this change, **some plugins will stop working unless you fix them**. Yours is one of them, so I've taken the liberty to fix it. Please take a look at the info we've compiled at <https://github.com/sketchplugins/cocoascript-migration/> for more info, and double check your scripts with [the latest beta](http://bohemiancoding.com/sketch/beta/) to make sure they are, indeed, fixed.

Thanks for your attention, and your contribution to the amazing Sketch Plugins community :)