add nb_inplace_divide for python 2

add nb_inplace_divide for python 2

Python 2's PyNumberMethods has nb_inplace_divide, but Python 3 does
not.  This patch adds it for Python 2.

This buglet didn't cause much fallout because the only non-NULL entry
in value_object_as_number after this is for valpy_divide; and the
missing slot caused it to slide up to nb_floor_divide (where
nb_true_divide was intended).

2016-05-24  Tom Tromey  <tom@tromey.com>

	* python/py-value.c (value_object_as_number): Add
	nb_inplace_divide for Python 2.