Fix install app version check

The app/plugin installer (OC_Installer::installApp()) does incorrect version checking. It should be updated to use the new version check method.

    It looks like this: https://github.com/owncloud/core/blob/master/lib/installer.php#L138
    It should look like this: https://github.com/owncloud/core/blob/master/lib/app.php#L226

    Someone mentioned this issue recently on the forums: http://forum.owncloud.org/viewtopic.php?t=9512&p=23701#p23701