Move from phpunit 5.7 to 7.3 and php 5.5 to 7.1

[jens1o]

preparing for PHP-FFMpeg/PHP-FFMpeg#412

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• Set declare(strict_types=1); in every file and fix any type-converting struggles.
• Use scalar type hints, null type hints and return type values(even void) in any classes.
• Check whether we have any workarounds for older php versions
  • Remove them if they aren't necessary anymore.
• Convert any array(...) to [...]

[jens1o]

maybe we can update to PHPUnit 7, too.

[jens1o]

26b9c3c

[peter279k]

Is any updated commit for this issue?

[jens1o]

No, not yet. However, if you want to help out, you're welcome to file a Pull Request. Personally I should have time again after the final exam on Monday for ~8 weeks then.

[rickmills]

Any plans to merge in the pull request from August above? This is starting to cause massive headaches now that everyone else has updated to Symfony 4 packages.

@jygaulier @nmaillat @jens1o