Set release date for 2.4

Author: Nudin

doc/vimwiki.txt

@@ -3000,7 +3000,7 @@ http://code.google.com/p/vimwiki/issues/list. They may be accessible from
 https://github.com/vimwiki-backup/vimwiki/issues.
 
 
-2.4 (not yet released)~
+2.4 (2019-03-24)~
 
 New:~
     * Add the option |g:vimwiki_text_ignore_newline|.