Use grpc 1.4.0

What does this MR do?

Update the grpc gem (used for Gitaly) to 1.4.0.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We don't want to fall behind in gRPC updates because the framework is still under a lot of development.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes omnibus-gitlab#2414 (closed)