support Boost 1.57.0

support Boost 1.57.0

Sometime after 1.55, boost introduced a forward declaration of
operator<< in optional.hpp. In 1.55 and earlier, when << was used
without the _io having been included, what got dumped was an implicit
bool conversion.

http://tracker.ceph.com/issues/10688 Refs: #10688
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
(cherry picked from commit 85717394c33137eb703a7b88608ec9cf3287f67a)

Conflicts:
	src/include/encoding.h
        trivial conflict