Update protobuf crate to 2.18.2

I would like to use `rules_rust_library` and `rules_rust_binary` to build Rust applications with Bazel.

I'm already using [rules-proto-grpc](https://github.com/rules-proto-grpc/rules_proto_grpc) to compile common proto files over different languages (C++, Python). As such, I would like to also use `rust_proto_library` from that repository to build the proto library, instead of the same rule from this repository.

[rules-proto-grpc](https://github.com/rules-proto-grpc/rules_proto_grpc) is already on protobuf crate version 2.18.2. In order to avoid incompatibilities between the crate version, it would be helpful to update this repository's dependencies. I have already updated the repo in a fork https://github.com/giokara/rules_rust and can open a PR with my changes.