set composer flags for lowest version to test

Author: nezaboravi

.travis.yml

@@ -5,8 +5,13 @@ php:
     - 7.3
     - 7.4
 
+env:
+    matrix:
+        - COMPOSER_FLAGS="--prefer-lowest"
+        - COMPOSER_FLAGS=""
+
 before_script:
-    - travis_retry composer update
+    - travis_retry composer update ${COMPOSER_FLAGS}
 
 script:
     - vendor/bin/phpunit