Update grpc to v1.12.3 in order to fix issues build issues due to the fact
that 1.12.1 doesn't contain node-pre-gyp as a bundled dependency.
See https://github.com/grpc/grpc-node/issues/365
The impetus is to improve error reporting for GRPC connection errors,
which was introduced in 1.8.4. Security and other fixes are included
as well.
https://github.com/grpc/grpc-node/releases