Also test lowest possible package combinations

Author: Toflar

.travis.yml

@@ -1,19 +1,29 @@
 language: php
 
-php:
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
+matrix:
+  include:
+    - php: 5.6
+    - php: 5.6
+      env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable'
+    - php: 7.0
+      env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable'
+    - php: 7.1
+    - php: 7.1
+      env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable'
+      env: COVERAGE='--coverage-clover build/logs/clover.xml'
+    - php: 7.2
+    - php: 7.2
+      env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable'
+  fast_finish: true
 
 sudo: false
 
 before_script:
-  - travis_wait composer install --no-interaction
+  - travis_wait composer update --no-interaction $COMPOSER_FLAGS
 
 script:
-  - mkdir -p build/logs
-  - php vendor/bin/phpunit --coverage-clover build/logs/clover.xml
+  - if [[ $COVERAGE ]]; then mkdir -p build/logs; fi
+  - php vendor/bin/phpunit $COVERAGE
 
 after_script:
-  - php vendor/bin/coveralls
+ - if [[ $COVERAGE ]]; then php vendor/bin/coveralls; fi

composer.json

@@ -18,7 +18,7 @@
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.6",
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "^5.7.25",
         "satooshi/php-coveralls": "2.0.x-dev"
     },
     "autoload": {