Prevent ext-event 2.4.0RC1 and use 2.3.0 instead
Prevent ext-event 2.4.0RC1 and use 2.3.0 instead

PHP 7.2 in combination with ext-event 2.4.0RC1 throws exceptions with invalid file descriptors, while everything works fine with 2.3.0 or PHP < 7.2.

This is a temporary fix to get the build green again.

Relates to #221.

amphp/amp