[MRG] FIX: Thresholded Nearest Centroid fails with non-encoded y

Minimal code:

    X = [[-2, -1], [-1, -1], [-1, -2], [1, 1], [1, 2], [2, 1]]
    y = ["r", "r", "r", "b", "b", "b"]
    clf = NearestCentroid(shrink_threshold=0.01)
    clf.fit(X, y)
    IndexError: arrays used as indices must be of integer (or boolean) type