Just uses github.com/japaric/trust to for continuous integration and
release building. trust uses github.com/rust-embedded/cross, which no
longer supports FreeBSD. (See github.com/rust-embedded/cross/issues/274
for details.)
Sadly, it's probably too much work to dive in and fix the FreeBSD image
for cross, so this commit disables the FreeBSD builds and releases on
Travis. I don't think we've ever seen a build failure on BSD only, so
losing the test coverage isn't a big deal.
It is unfortunate that we'll lose FreeBSD release builds though. I don't
think Just is in the FreeBSd ports tree, so it would be nice if someone
wanted to package it and get it in there.
