Require symfony/event-dispatcher >=2.4, since the library requires this

Author: Reinier Kip

.travis.yml

@@ -14,22 +14,23 @@ cache:
 before_script:
   - composer self-update
   - if [ "$SYMFONY_VERSION" != "" ]; then composer require --no-update symfony/config:${SYMFONY_VERSION} symfony/dependency-injection:${SYMFONY_VERSION} symfony/event-dispatcher:${SYMFONY_VERSION} symfony/http-kernel:${SYMFONY_VERSION} symfony/framework-bundle:${SYMFONY_VERSION}; fi;
+  - if [ "$SYMFONY_EVENT_DISPATCHER_VERSION" != "" ]; then composer require --no-update symfony/event-dispatcher:${SYMFONY_EVENT_DISPATCHER_VERSION}; fi;
   - composer install --no-interaction --prefer-source --dev
 
 script: phpunit --verbose
 
 matrix:
   include:
     - php: 5.6
-      env: [SYMFONY_VERSION="2.3.*"]
+      env: [SYMFONY_VERSION="2.3.*", SYMFONY_EVENT_DISPATCHER_VERSION="2.4.*"]
     - php: 5.6
       env: [SYMFONY_VERSION="2.7.*"]
     - php: 5.6
       env: [SYMFONY_VERSION="2.8.*"]
     - php: 5.6
       env: [SYMFONY_VERSION="3.0.*"]
     - php: 7.0
-      env: [SYMFONY_VERSION="2.3.*"]
+      env: [SYMFONY_VERSION="2.3.*", SYMFONY_EVENT_DISPATCHER_VERSION="2.4.*"]
     - php: 7.0
       env: [SYMFONY_VERSION="2.7.*"]
     - php: 7.0