Test PHP 7.4 on Travis CI

[WyriHaximus]

Required for #37, running into one failing test with it, only on PHP 7.4 due to GC improvements.

[WyriHaximus]

Interestingly enough #37 is fine, the issue is here already: https://travis-ci.org/reactphp/promise-timer/jobs/644555479#L552

[clue]

@WyriHaximus Interesting, the garbage references were believed to be fixed with #33/#34. I will look into the garbage references and what could cause a possible regression for PHP 7.4 only 👍

https://travis-ci.org/reactphp/promise-timer/jobs/644562579#L507

[WyriHaximus]

@clue Opened reactphp/promise#157 upstream, and there are two failing tests regarding to GC's there as well

[WyriHaximus]

@clue Also, this is the bug report I found yesterday when looking at PHP 7.4 specific GC changes: https://bugs.php.net/bug.php?id=72530

[clue]

@WyriHaximus Thanks for reporting this upstream in reactphp/promise#157 👍 I can not currently reproduce the problem upstream or here, so I think this is best addressed upstream first:

$ docker run -it --rm -v `pwd`:/data --workdir=/data php:7.4 php vendor/bin/phpunit

[WyriHaximus]

@clue yup, trying to find out why, but things seem to be point to the enqueue function

[WyriHaximus]

@clue@jsor Ok this is also green, took the same approach as reactphp/promise#157