Use Dependabot to automatically keep dependencies up-to-date

Quite a few dependencies in the `mix.exs` are out of date, including `phoenix` and `stripity_stripe`.

I built [Dependabot](https://dependabot.com) to automatically create dependency update PRs when new versions are released. It's totally free for open source (and personal accounts) and I'd love to get more people using it on Elixir projects. Would you be up for giving it a try? Zero config required - will take you 60 seconds I promise.

The PRs it created on my fork are [here](https://github.com/greysteil/code-corps-api/pulls).