Add requirements.txt for running on binder

[imagejan]

The possibility to run the notebooks from this repository on https://mybinder.org/ would make it even easier to follow those while reading the book.

To make them run on binder, it should be sufficient to add a requirements.txt or environment.yml file that declares the necessary dependencies.

[amueller]

It's on my todo for this week, but pull request also welcome ;)

[imagejan]

Done, PR #66 filed. I tested it on my fork with this URL:

https://mybinder.org/v2/gh/imagejan/introduction_to_ml_with_python/binder