Replace vim-gitgutter with vim-signify

vim-gitgutter is buggy. vim-signify might be better, it does allow
configuration of which events trigger a sign column update and this
patch takes advantage of that. Firstly, the `CursorHold` and
`CursorHoldI` events are disabled and replaced with `InsertLeave` and
`TextChanged` events. This makes the sign column updates immediate, and
since vim-signify is asynchronous it's faster too.

colors/fresh.vim

@@ -207,5 +207,9 @@ if has('gui_running') || &t_Co == 256
   call s:hi('SyntasticWarningLine', '', '', '')
   call s:hi('SyntasticError', '160', '', '')
   call s:hi('SyntasticWarning', '129', '', '')
+
+  call s:hi('SignifySignAdd', '2', '233', '')
+  call s:hi('SignifySignChange', '3', '233', '')
+  call s:hi('SignifySignDelete', '1', '233', '')
   "" }}}
 endif