[Themes] Remove Ruby from `theme push` command

[jamesmengo]

WHY are these changes introduced?

Fixes https://github.com/Shopify/develop-advanced-edits/issues/352

WHAT is this pull request doing?

• Removes
• Removes Ruby invocation for the theme push command

How to test your changes?

Post-release steps

Measuring impact

How do we know this change was effective? Please choose one:

• [ ] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• [ ] Existing analytics will cater for this addition
• [ ] PR includes analytics changes to measure impact

Checklist

• [ ] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [ ] I've considered possible documentation changes