Update Travis CI config file
Update Travis CI config file

Travis CI config file has been updated with the following main changes:

- Solidus and Ruby versions on Travis are updated to the lowest non-EOL
  version

- PostgreSQL and MySQL services has been explicitly added to fix errors
  due to their daemons not found

solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions