MRG : fix kfold balance due to int rounding

to avoid this:

```
>>> from sklearn import cross_validation
>>> list(cross_validation.KFold(14, 5, indices=True, shuffle=True,
random_state=32))
[(array([13,  2, 12,  9,  1, 10,  4,  3,  8,  6,  5, 11]), array([0,
7])), (array([ 0, 13, 12,  9,  1, 10,  4,  3,  8,  6,  5,  7]),
array([ 2, 11])), (array([ 0,  2, 12,  9,  1, 10,  4,  3,  6,  5, 11,
7]), array([13,  8])), (array([ 0, 13,  2, 12,  1, 10,  4,  3,  8,  5,
11,  7]), array([9, 6])), (array([ 0, 13,  2,  9,  8,  6, 11,  7]),
array([12,  1, 10,  4,  3,  5]))]
```