Update roundcube role to use wheezy backports

Debian 7.7 uses Roundcube 0.7.x, which is pretty old.  There appear to be lots of fixes and improvements in the 0.9.x and 1.0.x series.  Suggest moving to 0.9.x using the package in wheezy backports.

Changes:
- Added backports to apt configuration.
- Updated roundcube task to use the roundcube backport.
- Updated roundcube's main.inc.php configuration file to use the one included in the backport distribution.  Migrated configuration parameters from the existing configuration file except in cases where it looked like the Roundcube team was changing defaults that were not touched by Sovereign's configuration.  I may have gotten some wrong; needs review by a Sovereign maintainer.

This was tested in the development environment and has been deployed on my own server.