feat(symfony5): support Symfony5 and restore travis ci

[Oliboy50]

Why?

• we say in composer.json that we support >php7.0 but our bundle contains code only compatible with >php7.1 (nullable typing)... so we already dropped support for php7.0 💃
• our CI is failing for a while now... because it has been deactivated somehow 🤷‍♂
• we depend on LegacyEventProxy which is a Symfony @internal class... and has been brutally removed in Symfony 5

How?

• supports php7.1 to php7.3 in our CI (php7.4 may work fine but atoum is not compatible with it yet... so we can't test it well)
• enable support for Symfony 5 (we didn't test it in a real symfony5 project yet... but at least our tests passes 🤷‍♂) (see REAL TESTING section)
• drop support for the old EventDispatcherInterface::dispatch param orders
• drop support for unsupported symfony version https://symfony.com/releases
• drop support for symfony/config < 4.3 (because we use the new TreeBuilder::getRootNode() function which came with 4.2... but since 4.2 is not a supported version, we support only 4.3)

=> of course this will be a major update -> v3.0.0

REAL TESTING

I made a simple project to make sure the project does not throw any exception nor deprecation using Symfony5 => https://github.com/Oliboy50/sf5-testing