Full integration test that pulls in the ruby CLI and performs end to end test

[dmhenry]

See #257 and #259

• Validate package.json (at least existence if not some / all of its content)
• Validate presence of node_modules/ and that it is not an empty directory
• Validate that the merging for YAML/JSON works as expected
• Validate presence of extension.config.yml (possibly contents as well)
• Validate presence of .shopify-cli.yml