Fix compilation problem with GCC 4.9
GCC 4.9 causes build problem with boost::serialization.
http://stackoverflow.com/questions/23440210/error-no-match-for-operator-using-boostserialisation
Fixed by r1697
GCC 4.9 causes build problem with boost::serialization.
http://stackoverflow.com/questions/23440210/error-no-match-for-operator-using-boostserialisation