Ask for sudo when moving Caskroom.

[reitermarkus]

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Addresses Homebrew/homebrew-cask#24674, since people need to run sudo chown anyway, so might as well prompt them to.

@Homebrew/cask

[MikeMcQuaid]

Could you tell people to change it back afterwards? Also, how would you feel about printing a message with either the command to run and letting them do it themselves or, alternatively, the command you're about to run before you run it?

[vitorgalvao]

Brought me back to when we used to have to do something similar when using /opt.

[reitermarkus]

We could alternatively use sudo mv instead of chmodding /usr/local, this way we don't need to change the permissions back.