This PR switches the Nomad repository from using govendor to Go modules
for managing dependencies. Aspects of the Nomad workflow remain pretty
much the same. The usual Makefile targets should continue to work as
they always did. The API submodule simply defers to the parent Nomad
version on the repository, keeping the semantics of API versioning that
currently exists.

Always install go and vault: the check may accidentally use the CircleCI
pre-installed go version instead of the cached version.

Also, always install from sources without caching.  Go/vault
installation is basically a tarball download.  That's equivalent to
CirleCI caching without the complexity.  These steps don't add much time
either.

Lastly, infer the OS when downloading tarball to avoid managing them in
the job spec.

build darwin binaries in CI as a basic check.  Protects against
cross compile issues.