use alternate branch of d3 with generated files

Now using the bubble-default-on-mousedown-and-generated-files
branch of D3 which includes the generated d3.js content:

https://github.com/concord-consortium/d3/tree/bubble-default-on-mousedown-and-generated-files

Makes integration and demonstration easier.

But separate from the code branch to make the actual
code changes easy to review.

Also simplification of Makefile target for D3