Add migration message to shell handlers

[Rylan12]

Now that the functionality in this repo has been migrated to Homebrew/brew, we should update the shell handlers to print a message instructing users to migrate to the new handlers.

Now, the the following message whenever they're loaded to prompt the user to migrate:

Warning: Homebrew's command-not-found integration is out of date.
Please update your shell configuration to the new instructions printed by:
  brew command-not-found-init

[p-linnane]

Thanks for all the great work here!

[el7cosmos]

This hasn't been released yet on Homebrew/brew, the message keeps popping up and ain't no way to get rid of the message unless you disable the integration altogether

[Rylan12]

@el7cosmos this should be fixed now after a brew update