Update protoc dependency on protobuf to newest protobuf version (#189)

diff --git a/protoc_plugin/pubspec.yaml b/protoc_plugin/pubspec.yaml
index debcc92..4c14593 100644
--- a/protoc_plugin/pubspec.yaml
+++ b/protoc_plugin/pubspec.yaml
@@ -10,7 +10,7 @@
 dependencies:
   fixnum: ^0.10.5
   path: ^1.0.0
-  protobuf: ^0.12.0
+  protobuf: ^0.13.0
   dart_style: ^1.0.6
 
 dev_dependencies: