use md5sum-string instead of md5sum-sequence to adjust to upstream changes

https://github.com/marijnh/Postmodern/issues/27 has a good description
of the changes in the MD5 library:

The latest "release" of md5 (dated 20121125 according to quicklisp) will silently cast strings to byte arrays, yielding incorrect checksums and causing cl-postgres to fail to authenticate.