Add only clause to mix_test_watch

[johns10]

{:mix_test_watch, "~> 1.0", only: [:test], runtime: false}

Not necessary to include in production.

[dvcrn]

Thanks for that, you're welcome to send a small PR through to remove it :)

[johns10]

Done

[dvcrn]

Will get included in the next release, thank you ❤️