Trusty only on HHVM

.travis.yml

@@ -1,6 +1,3 @@
-dist: trusty
-sudo: required
-
 language: php
 
 php:
@@ -12,6 +9,13 @@ php:
   - 7.1
   - hhvm
 
+matrix:
+  include:
+    - php: hhvm
+      dist: trusty
+  allow_failures:
+    - php: hhvm
+
 before_script:
   - sh -c 'if [ "$TRAVIS_PHP_VERSION" != "hhvm" ]; then echo "session.serialize_handler = php" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi;'
   - composer install --dev --prefer-source