Temporarily allow failures of PHP 7.2RC6 build

See sebastianbergmann/phpunit#2853

.travis.yml

@@ -6,13 +6,16 @@ php:
 
 matrix:
   include:
+    - php: 7.2
     - php: 7.1
       env: dependencies="--prefer-lowest"
     - php: 7.1
       env: CHECK_CODESTYLE=1
       before_install: phpenv config-rm xdebug.ini
       script: composer check
       after_success: ~
+  allow_failures:
+    - php: 7.2
 
 install:
   - travis_retry composer self-update && composer --version