Update symfony/event-dispatcher version to ~2.8 or ~3.0

Currently this has no effect as tmdb/api requires ~2.0

Author: MarkRedeman

composer.json

@@ -14,7 +14,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "~4.0",
-        "symfony/event-dispatcher": "~2.0",
+        "symfony/event-dispatcher": "~2.8|~3.0",
         "illuminate/events": "~5.0",
         "doctrine/cache": ">=1.3.0",
         "monolog/monolog": ">=1.7.0"