Reload the solarized airline theme on the Colorscheme event.

This is just a copy of the function found in tomorrow.vim.
I'm using the following command in my .vimrc to fire the Colorscheme autocommand, in addition to toggling the background.
```vimscript
" Toggles the background color, and reloads the colorscheme.
command! ToggleBackground call <SID>ToggleBackground()
function! <SID>ToggleBackground()
    let &background = ( &background == "dark"? "light" : "dark" )
    if exists("g:colors_name")
        exe "colorscheme " . g:colors_name
    endif
endfunction
```