minor #31424 Fix tests (fabpot)

This PR was merged into the 3.4 branch.

Discussion
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Fix tests

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

As per https://www.patreon.com/posts/ubuntu-14-04-lts-26560500, Ondej's PPA does not support older versions of PHP anymore and so libsodium is now installed by Ubuntu directly, but the version is too old for PHP.

To make the tests pass again, I've removed libsodium from older versions of PHP and removed HHVM in the matrix as the tests do not pass for an obscure reason (no output); don't want to investigate what it could be.

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Commits
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6b63603 fixed tests

.travis.yml

@@ -20,9 +20,6 @@ env:
 
 matrix:
     include:
-        - php: hhvm-3.18
-          sudo: required
-          group: edge
         - php: 5.5
           env: php_extra="5.6 7.0 7.1"
         - php: 7.2
@@ -181,12 +178,6 @@ before_install:
               [[ -e $ext_cache ]] || (tfold ext.symfony_debug "cd src/Symfony/Component/Debug/Resources/ext && phpize && ./configure && make && mv modules/symfony_debug.so $ext_cache && phpize --clean")
               echo extension = $ext_cache >> $INI
           elif [[ $PHP = 7.* ]]; then
-              if ! php --ri sodium > /dev/null; then
-                  # install libsodium
-                  sudo apt-get install libsodium-dev -y
-                  tfold ext.libsodium tpecl libsodium sodium.so $INI
-              fi
-
               tfold ext.apcu tpecl apcu-5.1.16 apcu.so $INI
               tfold ext.mongodb tpecl mongodb-1.6.0alpha1 mongodb.so $INI
           fi