Relax the requirements on loss functiond derivatives.

We now require that the first derivative of the loss function
be positive only if the second derivative is non-zero. This is
because when the second derivative is non-positive, we do not use
the second order correction suggested by BANS and instead use
a simpler first order strategy which does not use a division by
the gradient of the loss function.

Change-Id: I3d65713f152611998e196ff389a7081acfdfd8c1