Add back Maxima 5.23.2 as an option.

This this is the version of Maxia that is readily available for RedHat
Enterprise Linux 6. Se the EPEL 6 row at
https://admin.fedoraproject.org/updates/maxima?_csrf_token=23e33dbe1600a6efb63bbabc3f4d18118045aa98

The OU is pretty well stuck with this version.