Set t_Co=256 forcely

I guess, most of users don't use 256 color environment, and vim-airline use background color with the numeric value that higher than 16. For example, my ubuntu box set t_Co to 8 in default. How about set t_Co=256 forcely?

When don't set t_Co=256 on gnome-terminal
![](http://go-gyazo.appspot.com/97f359ac9d60197f.png)

When set t_Co=256 on gnome-terminal
![](http://go-gyazo.appspot.com/a79c84baed07ed70.png)