[MRG] FIX: Bug in RFECV when step is not 1

Hi!

This patch fixes the bug with computation of the final `n_features_to_select` when `step` is not equal to 1. Everything should be clear from the code.

Please review.
Why is it 6 not 5? Shouldn't it be 10, 8, 6, 4, 2 ?