Bring template file over from `wp-cli/wp-cli`

[schlessera]

The Mustache template file required by this repository ( versions.mustache ) is still located in the wp-cli/wp-cli repository. This is an oversight from https://github.com/wp-cli/wp-cli/pull/3728.

It should be moved over to this repository and then later removed from wp-cli/wp-cli once a new release for this package was tagged and required in wp-cli/wp-cli.

• [x] Copy versions.mustache into wp-cli/core-command
• [x] Tag a new release of wp-cli/core-command with added file
• [ ] Add versions.mustache to inclusion list in utils/make-phar.php
• [ ] Require that release or higher of wp-cli/core-command in wp-cli/wp-cli
• [ ] Remove versions.mustache from wp-cli/wp-cli