chore: upgrade grpc to 0.6.4

apps/emqx_exhook/rebar.config

@@ -5,7 +5,7 @@
 ]}.
 
 {deps,
- [{grpc, {git, "https://github.com/emqx/grpc-erl", {tag, "0.6.2"}}}
+ [{grpc, {git, "https://github.com/emqx/grpc-erl", {tag, "0.6.4"}}}
 ]}.
 
 {grpc,