Phpunit 9 Compatibility

[artjomsimon]

• PHPUnit requires PHP 7.3 => dropped test in .travis.ci, added PHP 7.3 system requirement to composer.json
• The reference links to Sebastian Bergmann's original TAP printer are broken. I removed them, please feel free to add if you find a working reference link.
• ResultPrinter is now an interface, see Make PHPUnit\TextUI\ResultPrinter an interface sebastianbergmann/phpunit#4024
  The original Class we extend still exists, however it is marked @internal, so by definition we should now instead implement the Interface and reimplement a majority of the functionality in DefaultResultPrinter, which is also marked as @internal. I decided to not do that in this PR and instead continue extending the DefaultResultPrinter, even though it's marked as @internal.

Please have a look and see if anything can be improved, or if this can be merged.

[gh640]

@artjomsimon thanks for the PR. It looks fine and I've merged it. I released the version 5.0.0 and please try using it :)