Let's try to install it directly

.travis.yml

@@ -1,6 +1,9 @@
 language: php
 dist: trusty
 sudo: required
+env:
+  - HHVM=false
+  - PHPUNIT=phpunit
 
 matrix:
   include:
@@ -13,19 +16,20 @@ matrix:
     - php: 7.1
     - php: 7.2
     - php: hhvm
-      env: HHVM=true
+      env:
+        - HHVM=true
+        - PHPUNIT=./vendor/bin/phpunit
 
 install:
   - composer install
+  - if [[ $HHVM == true ]]; then composer require "phpunit/phpunit:5.7"; fi
 
 before_script:
   - mkdir -p build/logs
   - cd tests
 
 script:
-  - if [[ $HHVM == true ]]; then ./vendor/bin/phpunit --colors --coverage-clover ../build/logs/clover.xml .; fi
-  - if [[ $HHVM != true ]]; then phpunit --colors --coverage-clover ../build/logs/clover.xml .; fi
-
+  - $PHPUNIT --colors --coverage-clover ../build/logs/clover.xml .
 
 after_success:
   - cd ..