Installer: Do not use Vector skin if not installed
MediaWiki installer works fine in a bareskin
mode without Vector features, so do not
throw errors in the PHP error log just because
we don't have "skins" project extracted.
The message in the PHP error log:
Warning: include_once(w/skins/Vector/Vector.php): failed to open stream: No such file or directory in w/includes/installer/WebInstallerOutput.php on line 135
Stack trace:
1. {main}() w/mw-config/index.php:0
2. wfInstallerMain() w/mw-config/index.php:38
3. WebInstaller->execute() w/mw-config/index.php:79
4. WebInstaller->outputCss() w/includes/installer/WebInstaller.php:185
5. WebInstallerOutput->getCSS() w/includes/installer/WebInstaller.php:1200
Change-Id: I7b8bd77f5868af2ccf464e48db771f2e8e0472ff