Compatibility with scikit-learn 0.21

[ivanprado]

Some changes have been introduced to be compatible with scikit-learn 0.21:

• Fix Randomized(LogisticRegression|Lasso) test fail for scikit-learn 0.21 (related to https://github.com/TeamHG-Memex/eli5/pull/302/files)
• Fix xgboost installation failure by changing distro from trusty to xenial
• Fix legacy tox build by specifying the scipy version
• Fix init GradientBoostingClassifier changes introduced in scikit-learn 0.21 FIX several bugs in initial predictions for GBDT scikit-learn/scikit-learn#12983. Particularly, I think the conflicting change is that one: https://github.com/scikit-learn/scikit-learn/pull/12983/files#diff-2229647ab9a84dc25ab3d3d13800cb43R1654

[codecov-io]

Codecov Report

Merging #313 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   97.09%   97.16%   +0.07%     
==========================================
  Files          44       44              
  Lines        2856     2862       +6     
  Branches      542      544       +2     
==========================================
+ Hits         2773     2781       +8     
+ Misses         45       43       -2     
  Partials       38       38

Impacted Files	Coverage Δ
eli5/sklearn/explain_prediction.py	97.92% <100%> (+0.05%)	⬆️
eli5/sklearn/transform.py	100% <0%> (+5.71%)	⬆️

[lopuhin]

Looks great, thanks @ivanprado 👍

[kmike]

Thanks @ivanprado!