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
Merge "Installer: Do not use Vector skin if not installed"