Fix CircleCI configuration

[bonjourmauko]

Relates to #630

As discussed in #645 in this comment, this pull request fixes:

• The overlapping of caches between different python versions
• CircleCI workflow's job dependencies

Please note this is a small subset of #662

[fpagnoux]

Nice strategy to avoid duplication! Separating the caches for Python 2 and 3 was tricky, as Circle doesn't let use your own environment variable in the cache key 😞.

As discussed in Slack, and given that we seem to have something that seems to work right now, I'm in favor of delivering first, and refactoring after, as we don't know for sure what issues we'll face when we try to factorize the deployment step

[MattiSG]

I believe it's worth waiting for the dust to settle on openfisca/country-template#41 and then bring back knowledge to here 😉

[fpagnoux]

Superseded by #671.

Feel free to open another PR with the improvement you'll find relevant, if you wish 🙂