fixed dropping menu unit

Fixed issue with dropping li in .navbar .main. Now you can resize browser without any troubles.
![2014-03-05 19 15 48](https://f.cloud.github.com/assets/4189147/2335199/8d21c7b0-a481-11e3-88df-d3aef818ca05.png)
![2014-03-05 19 15 52](https://f.cloud.github.com/assets/4189147/2335203/92f9f630-a481-11e3-91a7-88b6ff3edf80.png)
![2014-03-05 19 15 57](https://f.cloud.github.com/assets/4189147/2335205/9ab17a74-a481-11e3-9ddc-ffc1ecea07e4.png)