[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.
He tries to reach exactly `n_features_to_select=1`, so it's 10, 8, 6, 4, 2 and 1. 

See [this](https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/feature_selection/rfe.py#L138) and [this](https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/feature_selection/rfe.py#L159) lines.