Avoid errors from older Vims that don't support <nomodeline>

The <nomodeline> flag for :doautocmd was implemented in 7.3.438. Certain
LTS distros, e.g. Ubuntu 12.04, come with even older Vim versions,
though.

Silently ignore the flag in such cases.

Closes https://github.com/mhinz/vim-signify/issues/185

autoload/sy.vim

@@ -96,7 +96,7 @@ function! sy#start() abort
 
   let b:sy.id_top = (g:id_top - 1)
 
-  silent doautocmd <nomodeline> User Signify
+  silent! doautocmd <nomodeline> User Signify
 endfunction
 
 " Function: #stop {{{1

doc/signify.txt

@@ -418,6 +418,8 @@ removes all the signs that Sy placed in the current buffer.  This may be
 useful if your want to use the values from `sy#repo#get_stats()` but do not
 want to see the signs.
 
+NOTE: This feature requires at least Vim 7.3.438 for |<nomodeline>|.
+
 ==============================================================================
 MAPPINGS                                                      *signify-mappings*