add flexibility on symfony dependencies

Author: Alexandre Guidet

composer.json

@@ -13,11 +13,11 @@
     ],
     "require": {
         "php": ">=5.3.0",
-        "symfony/console": "~2",
-        "symfony/process": "~2.6",
-        "symfony/config": "~2.6",
+        "symfony/console": "~2.6|~3.0",
+        "symfony/process": "~2.6|~3.0",
+        "symfony/config": "~2.6|~3.0",
         "cocur/slugify": "~1.0",
-        "symfony/yaml": "~3.1"
+        "symfony/yaml": "~2.6|~3.0"
     },
     "require-dev": {
         "phpunit/phpunit": "*"