remove composer --ignore-platform-reqs flag

Author: stefanotorresi

.travis.yml

@@ -10,7 +10,7 @@ cache:
 
 env:
   global:
-    - COMPOSER_ARGS="--no-interaction --ignore-platform-reqs"
+    - COMPOSER_ARGS="--no-interaction"
     - SITE_URL="https://zendframework.github.io/zend-diactoros"
     - GH_USER_NAME="Matthew Weier O'Phinney"
     - GH_USER_EMAIL="[email protected]"