Support Elixir 1.9 built-in releases

[rjanja]

Although Elixir 1.9 has built-in support for releases, Distillery is still necessary when using Bootleg to perform hot upgrades and when using overlays. In most other cases, the built-in release functionality should probably be selected.

Bootleg should probably continue to support Distillery as an optional dependency, detect its presence and behave accordingly with respect to generation of releases.