Create log directory when deploying

The log directory was created automatically by Capistrano 2, but not in version 3. Added a rake task that creates it before starting the app.

Warning: Not tested yet.
Seems like [best practices](https://github.com/capistrano/capistrano/search?q=execute&ref=cmdform) is to use a symbol (i.e. `:mkdir`) for the command. :smile: