fix dark mode in vim+iterm2

Before patch:
![1 mvim vim 2014-06-13 13-38-45 2014-06-13 13-38-49](https://cloud.githubusercontent.com/assets/122018/3267235/5ef70c62-f2c7-11e3-956b-949461034ba6.png)
After patch:
![1 mvim vim 2014-06-13 13-39-34 2014-06-13 13-39-38](https://cloud.githubusercontent.com/assets/122018/3267236/62089a56-f2c7-11e3-8ecc-0973e1387d27.png)

maybe it solves https://github.com/skwp/dotfiles/issues/461 too