upgrade protobuf version

pom.xml

@@ -202,7 +202,7 @@
             <dependency>
                 <groupId>com.google.protobuf</groupId>
                 <artifactId>protobuf-java</artifactId>
-                <version>2.4.1</version>
+                <version>2.6.1</version>
             </dependency>
             <!-- log -->
             <dependency>