Fix pill border overflow

When you have many Sensu APIs (more than 10), the data center list wraps but due to the active border, the LI menu doesn't render correctly:

![screen shot 2014-08-26 at 4 02 11 pm](https://cloud.githubusercontent.com/assets/465717/4046483/ada34fc4-2d32-11e4-86d3-13f7b6a30ce6.png)

This change fixes that

![screen shot 2014-08-26 at 4 01 51 pm png](https://cloud.githubusercontent.com/assets/465717/4046489/b909ef26-2d32-11e4-841b-36ff18cd792f.jpg)

The z-index is used to stop pills popping over the highlighted bored when hovered over.