#22 Soprano 2.7.0 and CMake 2.6

[Ralesk]

I recently tried to compile the newest Soprano without upgrading my CMake, and while it does run, it will never find (among other things) raptor2, and so never build the serializer and parser.

With CMake 2.8.4 it works, so I guess setting a minimum 2.8.0 in the CMakeLists.txt would be a good idea.