Use thin to start capybara server if available.

webrick sometimes raises following error in rspec:

    1) group collaborators as a group creator sign in I can add a collaborator
       Failure/Error: Unable to find matching line from backtrace
       RuntimeError:
         Circular dependency detected while autoloading constant AutocompleteController
       # ./app/middlewares/fallback_url_redirector.rb:9:in `call'