lnrpc: update grpc-gateway library to v2

docs/code_contribution_guidelines.md

@@ -433,12 +433,6 @@ _exact same_ version of `protoc`. As of the writing of this article, the `lnd`
 project uses [v3.4.0](https://github.com/google/protobuf/releases/tag/v3.4.0)
 of `protoc`.
 
-The following two libraries must be installed with the exact commit hash as
-described in [lnrpc README](https://github.com/lightningnetwork/lnd/blob/master/lnrpc/README.md)
-otherwise the CI pipeline on Travis will fail:
-- grpc-ecosystem/grpc-gateway
-- golang/protobuf
-
 For detailed instructions on how to compile modifications to `lnd`'s `protobuf`
 definitions, check out the [lnrpc README](https://github.com/lightningnetwork/lnd/blob/master/lnrpc/README.md).