Protobuf exact dependency

core/Cargo.toml

@@ -24,7 +24,7 @@ log = "0.4"
 num-bigint = "0.2"
 num-integer = "0.1"
 num-traits = "0.2"
-protobuf = "2.8.*"
+protobuf = "2.8.1"
 rand = "0.7"
 serde = "1.0"
 serde_derive = "1.0"