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.
Previous deps merge had been captured by an older version of godep,
updating godep and rerunning...
godep save $(go list ./... | grep -v /vendor/)
...yielded this diff.
New `godep` also produced a warning to pin to the major Go version
instead of the minor one, which we do here as well.