Fixed occasional package failure related to package "six"

Expected behavior

Expected behavior is the ability to install and import the package "spaCy" without any warning or errors.

Actual behavior

Occasionally, on installation and and import of the spaCy package, an error would occur related to the package six due to it's import of the package html5lib.

How to reproduce

Follow instructions here.

Solution

Create a requirements.txt file with html5lib, as completed in Pull Request #1126