Add missed test package

[ratson]

make test requires mock

[zsiciarz]

Hi r4ts0n and thanks for the PR! However the omission of mock is intended as I mentioned before in #35. I support Python 2.7 only because sometimes I have to, otherwise I do my development on 3. If you take a look at tox.ini, you'll see mock being installed for old Python versions. Perhaps I should update the development docs to be more explicit about Py3-first approach.

[ratson]

Understood, it would be great to have a Python 2 note about pip install mock in https://github.com/zsiciarz/django-envelope/blob/master/docs/development.rst

[zsiciarz]

If you have a note in mind, feel free to contribute it in a new PR. Otherwise I'll add a few words later today.

[zsiciarz]

FYI, I've added a paragraph about mock in bdb92a3.