Don't override standard GOV.UK drop down style

https://www.pivotaltracker.com/story/show/72270118

@futurefabric 
![screenshot 2014-06-03 18 16 14](https://cloud.githubusercontent.com/assets/63201/3163733/cf32dac4-eb42-11e3-807b-a0dbf90bcf57.png)