try another way to test php5.6

Author: xmarcos

.travis.yml

@@ -1,13 +1,21 @@
 language: php
 
-php:
-  - 5.6
-  - 7.0
-  - 7.1
+matrix:
+  include:
+    - php: 5.6
+      env: WITH_LOWEST=true
+    - php: 7.0
+    - php: 7.1
+
+before_install:
+  - composer validate
 
 install:
-  - composer install --dev --no-interaction --prefer-source
+  - if [[ "$WITH_LOWEST" == "true" ]]; then composer update --prefer-lowest; else composer install --prefer-source; fi
 
 script:
   - composer cs
   - composer test
+
+notifications:
+  email: false

composer.json

@@ -34,7 +34,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.8"
+        "phpunit/phpunit": "^5.7.19"
     },
     "scripts": {
         "test": "phpunit",