Fix #639: Use more careful way to enable numhl

The development versions of v0.3.2 also report `has('nvim-0.3.2')` to be
true even if they do not support the `numhl` feature. So here it catches
and ignores the possible errors.

autoload/gitgutter/highlight.vim

@@ -172,6 +172,7 @@ endfunction
 
 function! s:define_sign_linenr_highlights() abort
   if has('nvim-0.3.2')
+    try
       if g:gitgutter_highlight_linenrs
           sign define GitGutterLineAdded                 numhl=GitGutterAddLineNr
           sign define GitGutterLineModified              numhl=GitGutterChangeLineNr
@@ -187,6 +188,8 @@ function! s:define_sign_linenr_highlights() abort
         sign define GitGutterLineRemovedAboveAndBelow  numhl=
         sign define GitGutterLineModifiedRemoved       numhl=
       endif
+    catch /E475/
+    endtry
   endif
 endfunction