Add a region option for selecting EU region

[HoJSim]

You can set a region in config

config.action_mailer.delivery_method = :mailgun
config.action_mailer.mailgun_settings = {
		api_key: '<mailgun api key>',
		domain: '<mailgun domain>',
		region: 'eu',
}

[florianguenther]

this would be really helpful to have