Update composer.json

Author: crozet-magenta

composer.json

@@ -10,11 +10,11 @@
         "homepage": "https://github.com/rezzza/command-bus/contributors"
     }],
     "require": {
-        "php": ">=5.5.0",
+        "php": ">=7.1.0",
         "psr/log": "~1.0",
-        "symfony/event-dispatcher": "~2.5|~3.0|~4.0|~5.0",
-        "symfony/serializer": "~2.5|~3.0|~4.0|~5.0",
-        "symfony/dependency-injection": "~2.5|~3.0|~4.0|~5.0",
+        "symfony/event-dispatcher": "~2.5|~3.0|~4.0|~5.0|~6.0",
+        "symfony/serializer": "~2.5|~3.0|~4.0|~5.0|~6.0",
+        "symfony/dependency-injection": "~2.5|~3.0|~4.0|~5.0|~6.0",
         "doctrine/inflector": "~1.0"
     },
     "autoload": {