Add a workaround for a missing operator<< for gcc 5 on linux. See issue #205 for details.

include/date/tz_private.h

@@ -291,6 +291,7 @@ struct transition
     {
         using namespace date;
         using namespace std::chrono;
+        using date::operator<<;
         os << t.timepoint << "Z ";
         if (t.info->offset >= seconds{0})
             os << '+';