update to roundcube 1.0.3, and really update

Updating existing installed was broken. The new roundcube would be copied into a subdirectory of /usr/local/lib/roundcubemail.

Also fixes the image thumbnail issue raised on the forum (https://discourse.mailinabox.email/t/roundcube-thumbnails-not-showing/136), see http://trac.roundcube.net/wiki/Changelog.