Set PhpUnit version for older php

.travis.yml

@@ -12,20 +12,19 @@ cache:
     - $HOME/.composer/cache
     - $HOME/.cache
 
-php:
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
-
 matrix:
   include:
     - php: 5.4
-      env: COMPOSER_FLAGS="--prefer-lowest"
+      env:
+        - COMPOSER_FLAGS="--prefer-lowest"
+    - php: 5.4
+    - php: 5.5
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
 
 before_install:
   - sudo add-apt-repository ppa:mc3man/trusty-media -y
@@ -38,4 +37,4 @@ install:
   - composer update --prefer-dist $COMPOSER_FLAGS
 
 script:
-  - vendor/bin/simple-phpunit --verbose
+  - vendor/bin/phpunit --verbose