Make Cask::Artifact::Installer typesafe

[dduugg]

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Cask::Artifact::Installer instances would previously extend a module based on whether they were initialized with a manual flag. Instead, the flag is recorded and the logic is moved within the affected method (install_phase). I think this is much more idiomatic ruby, and it also removes the T.unsafe call (the TODO that suggests it could be removed no longer applied).

[MikeMcQuaid]

Looks good to me, thanks @dduugg!