Simplify code following code review.

theano/tensor/var.py

@@ -608,7 +608,7 @@ class TensorConstantSignature(tuple):
             # The following 2 lines are needede as in Python 3.3 with NumPy
             # 1.7.1, numpy.ndarray and numpy.memmap aren't hashable.
             if type(self._sum) is numpy.memmap:
-                self._sum = numpy.asarray(self._sum).sum()
+                self._sum = numpy.asarray(self._sum).item()
             if self.has_nan and self.no_nan.mask.all():
                 # In this case the sum is not properly computed by numpy.
                 self._sum = 0