Pin HHVM PHPUnit version

Author: GrahamCampbell

.travis.yml

@@ -4,21 +4,21 @@ jobs:
       dist: trusty
       language: php
       php: hhvm-3.18
-      before_install: composer self-update --2
+      before_install: composer self-update --2 && travis_retry composer require "phpunit/phpunit:^5.7.27" --dev --no-update -n
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: HHVM 3.24
       dist: trusty
       language: php
       php: hhvm-3.24
-      before_install: composer self-update --2
+      before_install: composer self-update --2 && travis_retry composer require "phpunit/phpunit:^5.7.27" --dev --no-update -n
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: HHVM 3.30
       dist: trusty
       language: php
       php: hhvm-3.30
-      before_install: composer self-update --2
+      before_install: composer self-update --2 && travis_retry composer require "phpunit/phpunit:^5.7.27" --dev --no-update -n
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.4