Fix major version of d3 to v2

The current version of Rickshaw has been thoroughly tested with
version 2 of d3 not with version 3 of d3.

Rickshaw unit tests are not compatible with the latest changes in d3.
See https://github.com/mbostock/d3/pull/1044 for discussion.

References:

- About tilde version ranges: https://npmjs.org/doc/json.html