Add missing six dependency

[sharkykh]

The code clearly uses six, so it should have it listed as a dependency.

[coveralls]

Coverage remained the same at 98.014% when pulling 809e175 on sharkykh:patch-1 into ca60ce6 on guessit-io:develop.

[Toilal]

Thanks, good catch (but still don't understand how build can pass without it ...)

[sharkykh]

but still don't understand how build can pass without it ...

I'm pretty sure Travis-CI has some version of six v1.11.0 pre-installed on Python builds.
Requirement already satisfied, skipping upgrade: six in /home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages (from guessit==3.0.5.dev0) (1.11.0)

Also, rebulk depends on it, so it could get installed from that:
Requirement already satisfied, skipping upgrade: six in /home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages (from rebulk->guessit==3.0.5.dev0) (1.11.0)