blast 2.2.28: use --without-gnutls to avoid compatibility error

Blast 2.2.28 is incompatible with newer versions of gnutls because
it uses constant GNUTLS_COMP_LZO, which was removed from gnutls.h
some time prior to gnutls 3.1.10. If gnutls is installed, it'll
be detected by blast's configure and break the build. Fix by
passing --without-gnutls to make blast ignore gnutls.

Fixes https://github.com/Homebrew/homebrew-science/issues/161.