Specify text color for GitHub's label


Before:
![2014-10-29 17 08 07](https://cloud.githubusercontent.com/assets/290782/4822511/c0c190d8-5f42-11e4-8222-e9fdce13dff1.png)
After:
![2014-10-29 17 08 00](https://cloud.githubusercontent.com/assets/290782/4822512/c0c23efc-5f42-11e4-8261-28e6e0066339.png)