Travis: fix PHP 5.5 builds

Travis PHP 5.5 builds are currently failing due to their using PHPUnit
5.x, which does not support PHP 5.5.

As a workaround, we manually install PHPUnit 4.x.

Fixes #20353

.travis.yml

@@ -8,6 +8,13 @@ language: php
 # Environment setup and test scripts execution
 #
 
+before_install:
+  # Workaround for failing PHP 5.5 builds, per https://github.com/travis-ci/travis-ci/issues/5206
+  - if [[ ${TRAVIS_PHP_VERSION:0:3} == "5.5" ]]; then
+        curl -s -o $HOME/.phpenv/versions/5.5/bin/phpunit https://phar.phpunit.de/phpunit-old.phar;
+        chmod +x $HOME/.phpenv/versions/5.5/bin/phpunit;
+    fi
+
 before_script:
   - ./scripts/travis_before_script.sh