Fix color scheme error by enabling 256 color.

Add directive in .vimrc: 'set term=xterm-256color'. 'set t_Co=256' also works, but I'm not sure if it compatible with Tmux. 

For reference: http://stackoverflow.com/questions/15375992/vim-difference-between-t-co-256-and-term-xterm-256color-in-conjunction-with-tmu