Upgrade protobuf to 2.6

I had to update protobuf definition for some internal experiments and found that working with protobuf 2.4 is rather inconvenient. It cannot be installed with brew on mac and building it on mac always result is build errors hence leaves an option of switching to linux to run protoc.

I decided to upgrade to 2.6 instead. It is compatible with 2.4 on wire and shaded so should not create any problems. All tests passed.

Please ignore changes in java files in attached patch during review; these are auto-generated.