Y culling doesn't seem to exist

I put a max value of 4 on y.tick.culling.max and it doesn't seem to modify the Y axis. The x axis culling seems to work fine.

``` javascript
var chart = c3.generate({
data: {
columns: [
['sample', 30, 200, 100, 400, 150, 250, 30, 200, 100, 400, 150, 250, 30, 200, 100, 400, 150, 250, 200, 100, 400, 150, 250]
]
},
axis: {
y: {
//type: 'category',
tick: {
culling: {
max: 4 // the number of tick texts will be adjusted to less than this value
}
// for normal axis, default on
// for category axis, default off
}
}
}
});
```