Fix axis for scales that don’t define ticks.

In addition to ordinal scales, quantile, quantize and threshold scales don’t define a ticks method. Unlike ordinal scales, however, these quantitative scales also don’t define a rangeBand method, so the axis was crashing trying to render these scales. This fixes #1367 by checking for the required rangeBand method rather than assuming scales without a ticks method define a rangeBand.

Note, however, that even with this change you probably still want to define tick values explicitly when using one of these scales with an axis. That’s because each tick in the axis represents a span of values in the domain; you can use the invertExtent method to get the span for each tick value.