Prop validation for axis fails with ordinal scales #36
Description
I'm getting the following prop validation failure when an xScale is ordinal:
Warning: Failed propType: Invalid prop `xScale` of value `band` supplied to `Grid`, expected one of ["linear","identity","sqrt","pow","log","quantize","quantile","ordinal","time"].
It looks like when a scale is set to ordinal, it actually is set to 'band' via axis.jsx: https://github.com/react-d3/react-d3-core/blob/master/src/axis/axis.jsx#L80
Would including 'band' in the allowed prop types fix this?