Advance protoc dependency version as well

src/objective-c/!ProtoCompiler-gRPCPlugin.podspec

@@ -95,7 +95,7 @@ Pod::Spec.new do |s|
   s.preserve_paths = plugin
 
   # Restrict the protoc version to the one supported by this plugin.
-  s.dependency '!ProtoCompiler', '3.0.2'
+  s.dependency '!ProtoCompiler', '3.1.0'
   # For the Protobuf dependency not to complain:
   s.ios.deployment_target = '7.1'
   s.osx.deployment_target = '10.9'