minor #4622 Disallow PHP 7.4 failures on Travis CI (julienfalque)

This PR was merged into the 2.15 branch.

Discussion
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Disallow PHP 7.4 failures on Travis CI

[PHP bug](https://bugs.php.net/bug.php?id=78575) has been closed. Looks like the actual bug was in `mikey179/vfsstream` (bovigo/vfsStream#204). The fix has been merged and released so we can now disallow Travis CI failures.

Commits
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5e93aa9 Disallow PHP 7.4 failures on Travis CI

Author: julienfalque

.travis.yml

@@ -164,6 +164,3 @@ jobs:
                     tags: true
             after_deploy:
                 - ./dev-tools/trigger-website.sh ${TRAVIS_TOKEN} ${TRAVIS_TAG}
-
-    allow_failures:
-        - php: 7.4snapshot