Add support for Scalingo

[nwittstruck]

Hi there,

thank you so much for this build pack. I've been using it for a while with Scalingo. Scalingo has added Scalingo-20 as their new default stack a while ago, which is no longer compatible with older prebuilt packages.

I've added support for Scalingo-20, it works just fine with the packages for gigalixir-20 and heroku-20.

Would you be interested in adding this? Otherwise I can just continue using my fork.

I've added a blog post on how to deploy phoenix to scalingo here.