Fixes nice() for sub-second scales

When the interval between ticks would be measured in milliseconds, calling `nice()` fails with the following trace:

```
Uncaught TypeError: Object #<Object> has no method 'floor' d3.v3.js:9186
scale.domain.skip.floor d3.v3.js:9186
d3_scale_nice d3.v3.js:6893
scale.nice d3.v3.js:9184
...
```

The `Object` is `d3_time_scaleMilliseconds`, which does lack `floor` and `ceil`.

This PR adds a test to catch this failure, and it adds `floor` and `ceil` to `d3_time_scaleMilliseconds`, which fixes the issue.