website/update-check.php:
- add check for 2.91 and recommend 2.92
  (but still keep 2.3.7 as latest version for users not using 3.0 beta)


git-svn-id: svn://svn.code.sf.net/p/postfixadmin/code@1708 a1433add-5e2c-0410-b055-b7f2511e0802