Make a single authoritative list of requirements

Make the readme and travis configuration both reference the `install_requires`
section of `setup.py` via a cleverly written `requests.txt` file. From
`requires.txt`:

    "Now your `pip install -r requirements.txt` will work just as before. It
    will first install the library located at the file path `.` and then move on
    to its abstract dependencies"

    See: https://caremad.io/2013/07/setup-vs-requirement/

Fix #19.