Update peep to version 2.0.

I wasn't clear enough so let me try to explain again:
 - I do include `peep.py` in `/bin/` and that's all that's needed to install `requirements.txt`
 - I agree that `peep` usage should be documented so our users do not use pip
 - Including `peep` in `requirements.txt` is only so that [requires.io](http://requires.io) can ping us with pull requests and emails when a new version of peep is out, as it's done with all other packages. I find this functionality useful (e.g. I saw Erik's tweet about the new peep but it was only after requires.io reminded me about it, that I actually send this pull request) and on the same time it doesn't hurt to reinstall peep with peep.