Update app.rb - sort cards by default/balance date

Sort the "cards" array by isDefaultCard and then by balanceDate. 
This will make it much easier to find your default card. Currently my default card is in the middle of the screens on the pebble and I have to scroll through several each time I open the app to get to the payment barcode.
This will show the default card as the first screen, and then the rest in order of most recent balanceDate.