Use v1.1.5 of go-msgpack/codec/codecgen, so go-msgpack codecgen matches
the library version.

We branched off earlier to pick up
https://github.com/hashicorp/go-msgpack/commit/f51b5189210768cf0d476580cf287620374d4f02
, but apparently that's not needed as we could customize the package via
`-c` argument.