Bug fix/update app bg

Fixing bug, where Marathon would not update running tasks in app list in background when changing number of instances in app modal:

[![Marathon showing more running tasks than actually running](http://cl.ly/image/1x1D0h0x180s/Image%202014-08-25%20at%2014.03.19.png)](http://cl.ly/image/1x1D0h0x180s/Image%202014-08-25%20at%2014.03.19.png)
Could be a static value on the `Marathon` object.