Use the new distillery.release command

Summary of changes

In the Phoenix walkthrough, we were using the following command: mix do phx.digest, release --env=prod For Elixir 1.9+, that command was failing. Since the new mix releases, the release command had to be changed to distillery.release. The walkthrough was still referencing the old command.

Checklist

[x] New functions have typespecs, changed functions were updated
[x] Same for documentation, including moduledocs
[x] Tests were added or updated to cover changes
[ ] Commits were squashed into a single coherent commit

Licensing/Copyright

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bitwalker / distillery