Add support for Opendkim on Travis

Unfortunately, adding the opendkim PHP extension makes PHP crash when running Composer.

@xdecock Can you have a look at it as it makes support for opendkim unusable for anyone using Composer?

you don't need to set ``COMPOSER_ROOT_VERSION`` (it is needed only when you have a circular dependency involving the root package. So symfony was using it because some of its dev dependencies depend on Symfony components too, and ICU was creating issues too with a circular dependency. There is nothing like that in swiftmailer)