Updated unit tests

[stephanvierkant]

I joined this library as a maintainer a while ago and I struggle to handle the Pull Requests because I my knowledge of all internals of this library is a bit limited. Therefor some extra unit tests are very helpful, to prevent regression.

This is a first step to improve tests for this library. In the feature, I'd like to see updated unit tests as a requirement to every PR.

What I've done so far:

• Upgraded Symfony requirement to supported versions: 3.4 (LTS) or ^4.1. Other versions don't get security updates to it's important to upgrade as soon as possible.
• Fixed some PHPUnit deprecations
• Added some columns to PostDatatable
• Added PHP 7.2 and 7.3 to Travis, remove PHP 7.1. since we don't support that version anymore.
• Used Flex to install specific Symfony version (Used Symfony Flex in favor of symfony/lts symfony/swiftmailer-bundle#293)