fixed code view line height for firefox

It's good in WebKit browsers, but with gecko ...
**without fix:**
![gogsfix-before](https://cloud.githubusercontent.com/assets/1295945/5261227/8d3c8958-7a17-11e4-8ec4-1bd5480befa7.png)
**with fix:**
![gogsfix-after](https://cloud.githubusercontent.com/assets/1295945/5261228/91e29916-7a17-11e4-9ca1-64bef82b3293.png)

Please check if it's valid LESS syntax, because I don't have a LESS compiler installed ...