Bumped Symfony requirement to 3.4

Author: javiereguiluz

composer.json

@@ -17,7 +17,7 @@
     "require": {
         "php"             : ">=7.1",
         "monolog/monolog" : "~1.6|~2.0",
-        "symfony/yaml"    : "^2.8|^3.4|^4.0|^5.0"
+        "symfony/yaml"    : "^3.4|^4.0|^5.0"
     },
     "autoload": {
         "psr-4": { "EasyCorp\\EasyLog\\": "src" }