boost/serialization/optional doesn't include necessary header

[ben.carlyle@…]

The following function refers to version<T>::value without an include of boost/serialization/version.hpp, resulting in user code that won't compile. It is possible to work around this problem by including boost/serialization/version.hpp before including boost/serialization/optional.hpp. I did not require a work-around with boost 1.32.0, but both 1.34.0 and 1.34.1 appear to need the work-around.

template<class Archive, class T>
void save(
    Archive & ar, 
    const boost::optional<T> & t, 
    const unsigned int /*version*/
)