Verson 3.20.0 for Hyrax 1.15.0

We now have debian packages for libdap, built using Travis CI

The library now returns only DAP 3.2 DDX responses, ending confusion about
what exactly the DDX syntax would be.

Using libtirpc (Libtirpc is a port of Suns Transport-Independent RPC library to Linux).
Thanks to Orion Poplawski.

Use quotes when testing $CC in the configure script.
This avoids the error "test: too many arguments" appearing in the
configure output if CC contains multiple words, such as "ccache cc".
Thanks to Ryan Schmidt.

Testing and coverage analysis.
Thanks to Ed Hartnett.