Fix for IE8

`Backburner#run` is broken on IE by IE specified code.
Therefore the many tests that related `Ember.run` are failed at Ember on IE8.

This patch makes all backburner's tests happy :smile: 

Before:
![2014-05-24 2 52 52](https://cloud.githubusercontent.com/assets/290782/3069957/1a2fba18-e2a3-11e3-87f3-3381a3928e0b.png)

After:
![2014-05-24 2 54 30](https://cloud.githubusercontent.com/assets/290782/3069985/53357a0a-e2a3-11e3-9deb-bc1a60c3b21f.png)