Fix L2 regularization order in sgd_fast
DOC explain labels parameter to confusion_matrix
corrected doctests after moving L2 penalty application in SGD
ENH speed up sklearn.feature_selection.chi2

New version is ~4× faster and reuses allocated arrays.
Merge remote-tracking branch 'upstream/master' into pr/2016
added SGD L2 fix to whatsnew
fix: add missing str formatting operator
DOC dependency installation with yum (Red Hat, CentOS)
FIX bug (swapped args) in chi2

Added a non-regression test.
FIX yet another chi2 bug
ENH remove pos_label argument with multilabel binary indicator format
ENH remove warnings at testing time
Documentation ERROR: mixture.DPGMM.precs_

in the code: mixture.DPGMM.precs_
in the doc: mixture.DPGMM.precisions_