Update dependency on github/golang/protobuf to latest.

For your convenience, the relevant commit list is
https://github.com/golang/protobuf/commits/master

I would like this since it would make it possible for me to link
against a modern timestamp.proto.

I changed the hash to the most recent change, but the change
that is most important to me is
github.com/golang/protobuf/commit/157d9c53be5810dd5a0fac4a467f7d5f400042ea
> regenerate pb.go files with protobuf 3.3.0

It would also be nice to have
github.com/golang/protobuf/commit/5a0f697c9ed9d68fef0116532c6e05cfeae00e55
> Deserialize JSON NaN, Infinity and -Infinity to corresponding Go values

Author: mikesamuel

go/deps.bzl

@@ -19,7 +19,7 @@ DEPS = {
     "com_github_golang_protobuf": {
         "rule": "new_go_repository",
         "importpath": "github.com/golang/protobuf",
-        "commit": "8ee79997227bf9b34611aee7946ae64735e6fd93", # ~ Nov 16, 2016
+        "commit": "748d386b5c1ea99658fd69fe9f03991ce86a90c1", # ~ Jul 26, 2017
     },
 
     "org_golang_google_grpc": {