minor #4345 Travis: PHP 7.4 isn't allowed to fail anymore (Slamdunk)

This PR was merged into the 2.12 branch.

Discussion
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Travis: PHP 7.4 isn't allowed to fail anymore

Following #4340, stated that currently `7.4snapshot` isn't failing and that the community seems to already trust the state of art of the PHP 7.4 branch (see e.g. [Ocramius/ProxyManager](https://github.com/Ocramius/ProxyManager/blob/2.3.1/composer.json#L22)), I suggest to start right now to require PHP 7.4 green builds

Commits
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1e3fa6d Travis: PHP 7.4 isn't allowed to fail anymore

Author: keradus

.travis.yml

@@ -165,6 +165,3 @@ jobs:
                     tags: true
             after_deploy:
                 - ./dev-tools/trigger-website.sh ${TRAVIS_TOKEN} ${TRAVIS_TAG}
-
-    allow_failures:
-        - php: 7.4snapshot