2.x support - crossfilter.quicksort removed in crossfilter2@1.5.0

[kuzjka]

dc 2.x declares major version dependency of crossfilter2: ^1.4.6
However, crossfilter2 1.5 has a breaking change. quicksort function is not used anymore (see Release notes ), but the function was exported and used by dc

Following error occurs with dc 2.2.2 and crossfilter2 1.5.2:

Uncaught TypeError: e.quicksort is undefined
    ordering dc.js:1342
    capMixin dc.js:4473
    rowChart dc.js:8595
    ...

I'll prepare a PR for 2.x to limit crossfilter2 dependency to 1.4.x as a quickfix for your consideration.