right protobuf-java version?

I think this needs protobuf-java > 2.4.1. I had to make this change to get it to build:

```
diff --git a/pom.xml b/pom.xml
index 483c876..f89ff55 100644
--- a/pom.xml
+++ b/pom.xml
@@ -275,7 +275,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
-      <version>2.4.1</version>
+      <version>2.5.0</version>
     </dependency>
     <dependency>
       <groupId>io.netty</groupId>
```