Fix 429587d: set default value for visualeditor-enable to 0, not 1

With our current config, 429587d would have enabled VisualEditor by
default on all wikis, which is not what we want to do. We can make the
-enable preference default to true in the extension if we really want
to, but that requires a change to wmf-config as well.

Change-Id: I95664588e5e4e3d6caed90e1c83accc9434ecd49