Update minimum protobuf version.

This compile issue https://code.google.com/p/protobuf/issues/detail?id=121 seems to affect node-mapnik trunk under GCC 4.4.3/linux, with errors like

    /usr/include/google/protobuf/repeated_field.h: In member function ‘void google::protobuf::internal::RepeatedPtrFieldBase::SwapElements(int, int)’:
    /usr/include/google/protobuf/repeated_field.h:630: error: no matching function for call to ‘swap(void*&, void*&)’