FIx issue #1290: pass arg is_fitted to super()

[stergion]

This PR fixes issue #1290

Passes constructor argument is_fitted of class SilhouetteVisualizer to super().

[codecov[bot]]

Codecov Report

Merging #1291 (a3812cb) into develop (40e0482) will increase coverage by 0.37%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1291      +/-   ##
===========================================
+ Coverage    90.88%   91.26%   +0.37%     
===========================================
  Files           93       84       -9     
  Lines         5301     4795     -506     
===========================================
- Hits          4818     4376     -442     
+ Misses         483      419      -64     

Impacted Files	Coverage Δ
yellowbrick/cluster/silhouette.py	98.43% <100.00%> (ø)
yellowbrick/anscombe.py
yellowbrick/text/umap_vis.py
yellowbrick/__init__.py
yellowbrick/download.py
yellowbrick/model_selection/rfecv.py
yellowbrick/base.py
yellowbrick/classifier/__init__.py
yellowbrick/features/manifold.py
yellowbrick/model_selection/validation_curve.py

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