Updated README with new protobuf version

Signed-off-by: James Turnbull <[email protected]>

Author: jamtur01

README.markdown

@@ -119,6 +119,6 @@ mvn deploy -rf :riemann-java-client
 
 # Hacking
 
-You'll need [protobuf 3.7.1](https://github.com/google/protobuf) - you
+You'll need [protobuf 3.16.1](https://github.com/google/protobuf) - you
 can just install the `protoc` binary. After that, `mvn package` should
 build a JAR, and `mvn install` will drop it in your local repository.