[MRG ] Fix issue #124

[glemaitre]

We were not selecting the samples from the good array.

I change this and enforce that any other implementation using the _selection_dist_based function is doing so. However, we cannot test this case since this is a class method.

[coveralls]

Coverage decreased (-0.03%) to 98.025% when pulling bddb621 on glemaitre:issue_124 into 19969f6 on scikit-learn-contrib:master.