Merge pull request #463 from wp-cli/theme-version

theme install: --version parameter ignored
contributing: use `behat --expand`

Without --expand, it's hard to debug failures in scenarion outlines.
behat: generate new plugin instead of relying on Hello Dolly

This allows us to check more things, since all the plugin data is frozen.
Ensure `wp plugin delete` and `wp theme delete` output consistent messages.
Merge branch 'behat-upgradables' into dev