make code consistent with erfcinv and erfinv
make code consistent with erfcinv and erfinv

niboshi/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,pfnet/chainer,wkentaro/chainer,hvy/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,okuta/chainer,tkerola/chainer,keisuke-umezawa/chainer,chainer/chainer,hvy/chainer,okuta/chainer