Fix fixed min samples

[deargle]

See #185 (but doesn't close that issue)

[deargle]

Also, setting tox to test all versions that I saw in .travis.yml

[codecov-commenter]

Codecov Report

Merging #194 into master will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   77.59%   77.80%   +0.21%     
==========================================
  Files          10       10              
  Lines         741      739       -2     
  Branches      156      155       -1     
==========================================
  Hits          575      575              
  Misses        134      134              
+ Partials       32       30       -2     

Impacted Files	Coverage Δ
kmapper/kmapper.py	88.44% <100.00%> (-0.10%)	⬇️
kmapper/plotlyviz.py	42.76% <0.00%> (ø)
kmapper/cover.py	89.77% <0.00%> (+2.27%)	⬆️

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