Update settings for new vim-gitgutter

New vim-gitgutter enables eager mode by default and follows Vim's
`updatetime` for purposes of timing.

dotfiles/vim/vimrc

 let g:gitgutter_sign_removed = '❌'
 let g:gitgutter_sign_removed_first_line = '^'
 let g:gitgutter_sign_modified_removed = '❰'
-let g:gitgutter_eager = 1
 
 let g:airline_theme = 'zenburn'