Only override the dev box image server if Vignette is disabled

Only override the dev box image server (via `wfReplaceImageServer`) if `$wgEnableVignette` is set to false. This will simplify the setup for vignette in development. You'll only have to set `$wgEnableVignette` and it will switch properly without `wfReplaceImageServer` interfering.

Related: https://wikia-inc.atlassian.net/browse/PLATFORM-513.

/cc @nmonterroso 
``strpos() !== false`` would be enough here :)