Recently we've added the possibility to run eZ Platform v2 and v3 on PHP 7.4. We should take advantage of that and start using it for tests.

We need to:

• switch the default version of PHP used on Travis to PHP 7.4 (for 2.5,3.0 and master)
• add nightly jobs running tests using on PHP 7.3:
  a) one nightly job in ezplatform metarepository (running all Behat tests for open-source)
  b) one nightly job in ezplatform-page-builder repository (running all Behat tests)

The second job should be part of a regression run, as PHP 7.3 is still the recommended one and we need to be sure it's working correctly (and nobody used 7.4-only features by accident).