Remove Docker references

[stephendonner]

We should update (remove) Docker-specific instructions and references, from at least here: https://github.com/mozilla-services/axe-selenium-python/blob/22a6e1e09a19b282b40729d81cec475c1baeb371/README.rst#contributing

[kimberlythegeek]

Good catch. thanks @stephendonner

[kimberlythegeek]

closing in favor of #136