This includes a bugfix to the cty/msgpack package to ensure correct
decoding of unknown and null values.
This also includes updates to cty's dependencies.
govendor's lack of understanding of transitive versions was making it hell
to figure out some dependency hell between etcd, grpc, and protobuf. After
fighting with it for a few hours, I decided to give Go 1.11rc2 a try since
previous experiments had been promising on the 1.11 tree in master.
The dependencies all worked out first time when managed using the Go
Modules code, and so we'll run with this now to continue to make progress
though we may wish to back out of this nearer to release and return to
govendor for a while until other projects have caught up.
However, since this commit includes a vendor directory produced using Go
Modules it doesn't actually _require_ Go 1.11 to build, and instead
requires it only to make further changes to the selected versions in the
vendor dir. Go 1.10's vendoring support will still find the modules in
their expected locations within the vendor dir.
