[3.6] Test on HHVM 3.30 too (#427)

Author: GrahamCampbell

.travis.yml

@@ -4,6 +4,14 @@ matrix:
       dist: trusty
       language: php
       php: hhvm-3.18
+      before_install: travis_retry composer require "phpunit/phpunit:^5.7.27" --dev --no-update -n
+      install: travis_retry composer install --no-suggest --prefer-dist -n -o
+      script: vendor/bin/phpunit
+    - name: HHVM 3.30
+      dist: trusty
+      language: php
+      php: hhvm-3.30
+      before_install: travis_retry composer require "phpunit/phpunit:^5.7.27" --dev --no-update -n
       install: travis_retry composer install --no-suggest --prefer-dist -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.4