Switch to phpunit 4.8 for PHP5.6 on Travis CI

.travis.yml

@@ -33,16 +33,11 @@ install:
         # symfony/console 4.0 (dependency of php-coveralls) relies on is_iterable()
         # function, added in PHP 7.1.0 and absent in HHVM
         composer require --no-update 'symfony/console:^3.0'
-
-        # htmlpurifier 4.10.0 is broken on HHVM
-        # https://github.com/ezyang/htmlpurifier/issues/169
-        composer require --no-update 'ezyang/htmlpurifier:dev-master#89b3fe4'
       fi
     - |
-      if [[ "${TRAVIS_PHP_VERSION}" = 5.3 ]]; then
-        # htmlpurifier 4.11.0 is incompatible with PHP < 5.4
-        # https://github.com/ezyang/htmlpurifier/pull/225
-        composer require --no-update 'ezyang/htmlpurifier:4.10.0'
+      if [[ "${TRAVIS_PHP_VERSION}" = 5.6 ]]; then
+        # /home/travis/.travis/functions: line 109:  4732 Segmentation fault (core dumped)
+        composer require --no-update 'phpunit/phpunit:^4.8'
       fi
     - composer install