Fix ValueError: Buffer dtype mismatch, expected 'Py_ssize_t' but got 'lo...

...ng' on win-amd64

E.g. on win-amd64-py3.4:
```
======================================================================
ERROR: test_slic.test_enforce_connectivity
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\nose\case.py", line 198, in runTest
    self.test(*self.arg)
  File "X:\Python34\lib\site-packages\skimage\segmentation\tests\test_slic.py", line 134, in test_enforce_connectivity
    convert2lab=False)
  File "X:\Python34\lib\site-packages\skimage\segmentation\slic_superpixels.py", line 193, in slic
    max_size)
  File "_slic.pyx", line 220, in skimage.segmentation._slic._enforce_label_connectivity_cython (skimage\segmentation\_sl
ic.c:3771)
ValueError: Buffer dtype mismatch, expected 'Py_ssize_t' but got 'long'
```