Add _vendor to the end of GOPATH instead of the beginning.

[bdarnell]

Pass an explicit path to protoc-gen-gogo.

Both of these changes are motivated by compatibility with the emacs go-projectile package and supporting an isolated per-project GOPATH.

@tschottdorf @cockroachdb/developers

[tbg]

LGTM.
btw, should we also look at vendoring via godep -r and/or become go-gettable (that should work if we just call make for our C++ deps via go generate)?

[bdarnell]

go get will not call go generate (go generate is designed to generate files which you then check in to source control), so we won't be able to become go-gettable. I first looked at vendoring via submodules (using godep would be similar), but hit a snag in the docker build. We get into trouble because _vendor includes both source and binaries, and we treat _vendor/rocksdb as a special case to speed up the build.