FIX: Canny label strel now 8-connected for all architectures

We found this bug in scikit-image a bit back, when testing the scipy.ndimage.label changes in 0.13dev. Turns out `np.ndarray((3, 3), bool)` does different things pseudo-randomly on different architectures, so it's a poor choice of a structuring element. This fix forces it to use an 8-connected neighborhood regardless of architecture.

For example, on my OSX Canopy install I get True in upper left and lower right, otherwise False... while on Win7 everything is True.

My suspicion is the original developer worked on a system where this reliably returned all True, but that just isn't the case generally. In any event, this fixes the problem.