Fix error with six dependency

Both CherryPy (8.1.2) and fs (2.0.0) depend on six. It would seem that CherryPy will just accept the latest version, while fs requires version 1.10.0, but because CherryPy is the first dependency install_dependencies.sh is installing 1.11.0 and then printing an error because that version is incompatible with fs 2.0.0.