closes#38
Added a preprocessor define CEREAL_OLDER_GCC that exists if using
any GCC 4.7.x or earlier. If this is defined some type traits change to
our original solution (prior to VS compatible), which is probably what we'd switch
to if VS ever gets around to fully supporting SFINAE and constexpr.
In map and set, as well as in unit tests, use insert instead of emplace for 4.7.x.
Shane Grant