[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.
We need to divide `n_features` - 1 by `step`. So if `n_features` - 1 is divisible by `step` then the answer is 1 + (`n_features` - 1) // `step`. If not, the answer is 1 + (`n_features` - 1) // `step` + 1. These two cases can be combined together like this:

1 + (`n_features` + `step` - 1) // `step`. 

Does it look correct to you?