Do not set Capybara.app_host if it is not nil (already set)

Prevent overriding of Capybara.app_host by sauce gem, if it was already set (e.g. in spec_helper.rb, while configuring Capybara default options). Capybara.app_host need to be defined on a global scope in order to easily swap between different capybara drivers (:sauce, :chrome, etc)