Fix phpunit installation for travis

.travis.yml

@@ -1,19 +1,16 @@
 language: php
+dist: trusty
 php:
   - "7.4"
   - "7.3"
   - "7.2"
   - "7.1"
   - "7.0"
+  - "5.6"
+  - "5.5"
+  - "5.4"
 
 install:
   - sudo apt-get update
   - travis_retry composer self-update && composer --version
-  - travis_retry composer clear-cache
-  - export PATH="$HOME/.composer/vendor/bin:$PATH"
-  - travis_retry composer install --prefer-dist --no-interaction --no-progress
-  - travis_retry composer require --dev phpunit/phpunit:^6.0 --prefer-dist --no-interaction --no-progress
-
-script:
-  - php -doutput_buffering=On vendor/bin/phpunit
-
+  - travis_retry composer install --no-interaction --no-progress

composer.json

@@ -28,8 +28,8 @@
     }
   },
   "autoload-dev": {
-      "psr-4": {
-          "tests\\": "tests"
-      }
+    "psr-4": {
+      "tests\\": "tests"
+    }
   }
 }