Update documentation and requirements*.txt after 2.6 ordereddict change

[gsnedders]

As I commented in #177, the changelog (adding a dependency is a pretty major change!), readme (because it's now wrong!), and requirements*.txt files (because they don't require ordereddict on 2.6!) all need updating.

[gsnedders]

I'm also unconvinced this is any more "required" than lxml, etc. None of the "core" APIs break without it installed.

[Ivoz]

@gsnedders it does manage to randomly break stuff without notice: https://travis-ci.org/pypa/readme/jobs/80119096#L172