Remove Grunt, use WP-CLI to generate POT

[nathanrice]

We're trying to move away from Grunt. Genesis uses WP-CLI as the package that generates POT files for projects, so we should use that here too.

[nickcernis]

Reference for more info: https://github.com/studiopress/simple-urls/issues/23

How WP-CLI was implemented in Simple Hooks: https://github.com/studiopress/genesis-simple-hooks/blob/d0153b96242fe0b149c35d29272fa87b1bac8513/composer.json#L27