Restore boost 1.55 compatibility in pybind11 support.
Restore boost 1.55 compatibility in pybind11 support.

`boost::intrustive_ptr::reset(T * r, bool add_ref)` was added in boost
1.56. Use of this overload, though helpfully explicit, breaks
compatibility with older boost versions. For example, 1.55 is the
default install in debian 8 and is widely used.

`reset(t, true)` is equivalent to `reset(t)`, which can be used to
preserve backward compatibility. For details see:

http://www.boost.org/doc/libs/1_55_0/libs/smart_ptr/intrusive_ptr.html#Synopsis
http://www.boost.org/doc/libs/1_56_0/libs/smart_ptr/intrusive_ptr.html#Synopsis

ndarray/ndarray,ndarray/ndarray