Fixed 'currentWhen' depreciation warnings

The use of currentWhen is now depreciated in newer versions of Ember (1.8 and up): emberjs/ember.js@bd4de63dd9618cac29d8f07e733db02e29e458d7

I updated the code to favor `current-when` instead of `currentWhen` in order to satisfy the new requirements. In addition I tested the code with the tests provided and everything checks out.

Cheers!