MAINT Update CI

[chkoar]

• Remove pandas from ubuntu based py35 builds
• Update a failed test of InstanceHardnessThreshold that uses GradientBoostingClassifier. Changes have been made in scikit-learn so the classifier was returning different results. I used Naive Bayes instead.
• Update doctest for RUSBoostClassifier due to changes in trees in scikit-learn
• Remove Appveyor

[codecov]

Codecov Report

Merging #679 into master will decrease coverage by 2.18%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #679      +/-   ##
=========================================
- Coverage   98.59%   96.4%   -2.19%     
=========================================
  Files          82      82              
  Lines        4841    4842       +1     
=========================================
- Hits         4773    4668     -105     
- Misses         68     174     +106

Impacted Files	Coverage Δ
...election/tests/test_instance_hardness_threshold.py	100% <100%> (ø)	⬆️
imblearn/keras/tests/test_generator.py	8.62% <0%> (-91.38%)	⬇️
imblearn/tensorflow/_generator.py	30% <0%> (-70%)	⬇️
imblearn/keras/_generator.py	50.74% <0%> (-47.77%)	⬇️

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[glemaitre]

LGTM