Included 'start_label' for 'slic' segmentation.

[RobertSamoilescu]

The latest releases of skimage updated the slic default indexing to start from 1 instead of 0 for convention reasons. This conflicts with the alibi expectation which assumes that the indexing starts from 0.

This PR addresses this issue by forcing the slic segmentation indexing to start from 0 through the argument start_label=0. The default slic kwargs were updated with start_label=0 as well as the arguments in the example notebook. The docstring were updated as well to describe alibi expectation of the segmentation indexing. Those updates required a bump in the sklearn verison to a lower bound version of 0.17.2 (the stable version of 0.17.X).

Addresses issue #748

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[codecov]

Codecov Report

Merging #753 (61af313) into master (5f7b505) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
- Coverage   79.65%   79.63%   -0.02%     
==========================================
  Files         107      107              
  Lines       12657    12657              
==========================================
- Hits        10082    10080       -2     
- Misses       2575     2577       +2     

Impacted Files	Coverage Δ
alibi/explainers/anchors/anchor_image.py	93.12% <ø> (ø)
alibi/explainers/ale.py	65.21% <0.00%> (-0.44%)	⬇️
alibi/explainers/partial_dependence.py	45.67% <0.00%> (-0.21%)	⬇️

[jklaise]

LGTM!