Cannot pass functions to xAxis min/max

[narenranjit]

https://github.com/forio/contour/blob/3dd5f7ddfe42d74b82194fa3d176e735c8a952fb/src/scripts/core/cartesian.js#L218-L227

That function is correctly being called for each render, but it's only reading options the first time.

@jaimedp