Add property for protobuf artifact and upgrade to 2.5.0

As proposed by Geoffrey, moving from protobuf 2.4.1 to 2.5.0 will allow to benefit on a bundle archive for OSGI platform (felix, karaf, eclipse).

The existing modification includes : 
- Add property for protobuf of Google
- Change property version from 2.4.1 to 2.5.0