Downgrade grpc + tools to 1.30.2
This reverts commit f2767074 (but I now specify 1.30.2 in gemspec instead of 1.19 for clarity, though semantically it's the same change as we were matching via ~> on the minor version level)
It was causing problems on older CPU architectures due to new instructions being used in the shared library.
Edited by Matthias Käppler