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.

plugin/settings.vim

@@ -162,11 +162,6 @@ set mouse=a
 "   q - allow formatting with 'gq'
 set formatoptions+=rq
 
-" Always show the signcolum
-if exists('&signcolumn')
-  set signcolumn=yes
-endif
-
 " Enable modeline
 set modeline
 

vimrc

@@ -70,6 +70,9 @@ let g:ale_cmake_cmakelint_options =
 
 " Version control differences in the sign column
 Pack 'mhinz/vim-signify'
+if exists('&signcolumn')
+  set signcolumn=yes
+endif
 
 " format.vim - format with text objects
 Pack 'git@bitbucket.org:infektor/format.vim.git'