Use tuple instead of generator for company keys

[cuducos]

Fix #235

[augustogoulart]

I reproduced the bug and this solution has worked fine for me.

[jtemporal]

Great! Just tested it and it works perfectly! Thanks @cuducos ❤

@augustogoulart thank you also for the feedback! Much appreciated!

Just for documentation, how I tested this:

• since I don't work with a fork I could only update my local clone by doing: $ git pull
• checked out the changes by using: $ git show
• tried to run the command that was breaking before: $ python manage.py companies contrib/sample-data/companies.xz (no errors here o/)
• also ran the tests:

  $ python manage.py check
  $ python manage.py test
  $ yarn test 

  little note here: it wasn't really necessary to run yarn tests but I did just in case

Since all the steps above ran without errors I'm merging this 🎉