Dependencies version aren't fixed

[martinvol]

Issue Summary

A short summary of the issue

pixax-stripe uses stripe>=2.0 as dependency, as defined here. This causes that deployments of the code without changing dependencies could break because they are incompatible with the deployed code.

Steps to Reproduce

It can't be downloaded and reproduced, because your project will break as stripe changes they Python Library API.

In my case, the issue caused the following problem

1. Use stripe.CardError
2. Someday after a deployment, my server re-installed the dependencies and I got AttributeError: module 'stripe' has no attribute 'CardError'

What were you expecting to happen?

Every version of pinax-stripe should be expected to work with only one version of stripe.

What actually happened?

[blueyed]

I think the problem is that it was not pinned previously, which was done in https://github.com/pinax/pinax-stripe/commit/bb25a054eafaeaf08397048d43e5520ef26f8e0c.

Maybe you updated stripe before pinax-stripe already? Would be good to have your full traceback to see if it is from pinax-stripe, or maybe even your own app.

I think this issue can be closed, with further discussion in #584.