Revert "forge is down: switch temporarily to cached ounit"
(commit caf7cd560b8d6fc18177cb45c122f15f3ca098ba)

This is preferable as it makes it clearer that this isn't a patched version of
ounit. Moreover, the checksums between the official release and the
currently-used tarball differ.
Merge pull request #1912 from nbraud/ounit

Use the official tarball release for oUnit.