Remove sf 2.8 support

DoctrineMongoDBBundle will only support php 7.2, sf3.4 and up

Author: arendjantetteroo

composer.json

@@ -16,8 +16,8 @@
         "php": "^7.1",
         "ext-mongodb": "*",
         "mongodb/mongodb": "^1.4",
-        "symfony/console": "^2.7|^3.3|^4",
-        "symfony/yaml": "^2.7|^3.3|^4"
+        "symfony/console": "^3.4|^4",
+        "symfony/yaml": "^3.4|^4"
     },
     "require-dev": {
         "phpunit/phpunit": "^7.0",