Update libsodium version
Include configuration entries missing from bf96a1b3
Rework automated testing setup

New features in this branch require an unreleased version of
libsodium. In the process of tweaking the build setup to use the latest
version on `master`, it made sense to *always* test against a local
build of libsodium.

This commit also cleans up the Rake tasks involved somewhat, and removes
a warning caused when building against newer versions of MiniTest.