Add dependency on @grpc_ecosystem_grpc_gateway to repositories.bzl

[stevewolter]

bazel/proto_k8s_service.bzl depends on grpc_ecosystem_grpc_gateway, so we should pull it in in repositories.bzl.

There is some complexity, as if we use go_repository() we introduce a dependency on rules_go, but if we use git_repository() then projects that use Golang may have to override it in WORKSPACE before calling cloud_robotics_repositories to be able to depend on go_library rules in @grpc_ecosystem_grpc_gateway.