Remove dependency on d3

[tomwayson]

There are a few places where D3 is used but not needed. Need to make sure we only use d3 where we need to. Ideally, this library would depend on just vega, or once Cedar uses Rollup/ES2015, perhaps bring in only the bits of d3 (v4) that are needed via tree shaking.

[benstoltz]

Also #204

[benstoltz]

Also Vega is not up to date with d3 (v4).

[tomwayson]

That's fine. We don't need vega to be on v4, we can update our own d3 dep to v4 to tree shake out and bundle (w/ Cedar code) just what we need (if, in fact, we need anything at all).

[tomwayson]

I've looked through the source, and we only use d3.xhr(), d3.select(), and d3.entries() all of which can be done w/o d3.

[benstoltz]

@tomwayson Absolutely. We can use vega.utils for d3.xhr()