Updating demo datasets to retain column names

[angela97lin]

Closes #214

[codecov]

Codecov Report

Merging #223 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   96.56%   96.57%   +<.01%     
==========================================
  Files          92       92              
  Lines        2416     2422       +6     
==========================================
+ Hits         2333     2339       +6     
  Misses         83       83

Impacted Files	Coverage Δ
evalml/demos/diabetes.py	100% <100%> (ø)	⬆️
evalml/demos/wine.py	100% <100%> (ø)	⬆️
evalml/demos/breast_cancer.py	100% <100%> (ø)	⬆️

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[kmax12]

LGTM. We should update these methods further to include the proper target names after @dsherry finishes #215

[angela97lin]

All right; made a separate issue (#224) to follow up on this, merging and closing this for now :)