Prevent sticky type checking (E706) errors with older Vims (#681)

Vim used to throw the E706 error if you tried to change a variable's
type. This error can be triggered by vimwiki if, during the user
settings loop in `read_global_settings_from_user()`, the variable type
for `users_value` changes.

The vim error was removed in v7.4.1546 as part of
f6f32c38bf.

doc/vimwiki.txt

@@ -3313,6 +3313,7 @@ Contributors and their Github usernames in roughly chronological order:
     - Patrik Willard (@padowi)
     - Steve Dondley (@sdondley)
     - Alexander Gude (@agude)
+    - Jonny Bylsma (@jbylsma)
 
 
 ==============================================================================
@@ -3331,6 +3332,7 @@ New:~
     * PR #689: Allow |vimwiki-option-diary_rel_path| to be an empty string.
     * PR #683: Improve layout and format of key binding documentation in
       README and include note about key bindings that may not work.
+    * PR #681: Prevent sticky type checking errors for old vim versions.
     * PR #675: Add option |vimwiki-option-name| to assign a per wiki name.
     * PR #661: Add option |g:vimwiki_auto_header| to automatically generate
       a level 1 header for new wiki pages.