FIX: Reorder yarn/bundle install for compatibility (#642)

FIX: Reorder yarn/bundle install for compatibility (#642)

Some existing templates (e.g. those written by following [the multisite guide](https://meta.discourse.org/t/multisite-configuration-with-docker/14084) use the `after_bundle_exec` hook to run application code like `multisite:migrate`. Relatively recently, the ruby app has started depending on `yarn` dependencies for basic functionality, and therefore requires `yarn install` to have been run to avoid errors.

This commit moves the `yarn install` step before `bundle install`, so that existing users of the `after_bundle_exec` hook do not need to update their templates.