untangle.py: add support for v13.1 download URLs.

In version 13.1 of Untangle, the backup download URL has changed. That
means we need to support a new, non-integer version number "13.1". This
commit changes the version parameter from an integer to a string, and
adds support for two new version strings: "13" and "13.1".

Since the download URL was unchanged in v13.0, the existing routines
are used for that version. However, a new get_backup_v13_1() function
was added for v13.1. The only difference between that function and the
get_backup_v10() function is the word "webui" which has been changed
to "admin".

Version "13.1" is now the default version if none is specified.