bin/mixer_codegen.sh appears to download a bunch of stuff, it's unclear why it would vs using vendor/

I realize the script predates vendor so that's probably why - or I am missing why it is needed (cc @theganyo ) and I mistakenly made it worse with a recent PR (or again I don't understand the issue)

I think
https://github.com/istio/istio/blob/master/Gopkg.toml#L26
should bring down protos too ?
will test / filled golang/dep#1720