minor #28288 [travis] disable symfony/flex during phpunit install (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[travis] disable symfony/flex during phpunit install

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Fixing https://travis-ci.org/symfony/symfony/jobs/421206649

Commits
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4623fe74ca [travis] disable symfony/flex during phpunit install

.travis.yml

@@ -221,7 +221,11 @@ install:
               ([[ $deps ]] && cd src/Symfony/Component/HttpFoundation; composer config platform.ext-mongodb 1.5.0; composer require --dev --no-update mongodb/mongodb)
           fi
           tfold 'composer update' $COMPOSER_UP
-          tfold 'phpunit install' ./phpunit install
+          if [[ $TRAVIS_PHP_VERSION = 5.* || $TRAVIS_PHP_VERSION = hhvm* ]]; then
+              tfold 'phpunit install' 'composer global remove symfony/flex && ./phpunit install && composer global require --no-progress --no-scripts --no-plugins symfony/flex dev-master'
+          else
+              tfold 'phpunit install' ./phpunit install
+          fi
           if [[ $deps = high ]]; then
               echo "$COMPONENTS" | parallel --gnu -j10% "tfold {} 'cd {} && $COMPOSER_UP && $PHPUNIT_X$LEGACY'"
           elif [[ $deps = low ]]; then