You're listing libsodium versions 0.15 and 0.13 there, but it should
be 1.0.15 and 1.0.13.
Maybe for everyone's reference: Ubuntu 16.04 LTS comes with no
libargon2, and with libsodium 1.0.8; 18.04 LTS comes with libargon2
and with libsodium 1.0.16.
Debian Stretch has 1.0.11, and 1.0.17 in backports.
I was originally planning to just do this since I thought it'd be
non-controversial, but while implementing I discovered some gotchas, so I'm
putting it to the group for discussion. Please give attention to the
"Backward Incompatible Changes" section.