Pin versions of libraries and tools required for proto generation. (k…

…ubeflow#492)

This change pins the versions of the libraries that were used to
generate the proto definitions using dep. The Makefile is then modified
so that the tool and library versions used to build the proto generated
files are from the vendor directory. This is a hacky, short-term
solution to ensure a reproducible build while we work on switching to
bazel.

The versions in the Gopkg.toml file were chosen based on my experiments
that generated proto files that did not change from what is already
checked in.