fix tab css

The tab CSS is a bit buggy.

1. There is a white line below the active tab
2. The tabs jump by 1px when you switch between them

![image](https://cloud.githubusercontent.com/assets/494927/4736048/7e991f1e-59e8-11e4-8f48-f7777fb66eac.png)

This PR is just a fairly small CSS change that fixes these issues.