Update roundcube to 1.2.6.

RELEASE 1.2.6
-------------
- Don't ignore (global) userlogins/sendmail logging in per_user_logging mode
- Managesieve: Fix AM/PM suffix in vacation time selectors
- Fix bug where comment notation within style tag would cause the whole style
  to be ignored (#5747)
- Fix bug where it wasn't possible to scroll folders list in Edge (#5750)
- Fix addressbook searching by gender (#5757)
- Fix SQL syntax error on MariaDB 10.2 (#5774)
- Fix bug where it wasn't possible to set timezone to auto-detected value
  (#5782)
- Fix uninitialized string offset in rcube_utils::bin2ascii() and make sure
  rcube_utils::random_bytes() result has always requested length (#5788)
- Fix potential XSS vulnerability with malformed HTML message markup

(taca)