Update composer.json

Update twig requirement to 1.15 to support round filter used in https://github.com/sonata-project/SonataAdminBundle/blob/453c87199286d30f4918a06a2665ecbc7ed1e2b6/Resources/views/Core/add_block.html.twig#L3

composer.json

@@ -27,7 +27,7 @@
         "symfony/twig-bridge": "~2.3",
         "symfony/class-loader":"~2.3",
         "sensio/generator-bundle": "~2.3",
-        "twig/twig": "~1.12",
+        "twig/twig": "~1.15",
         "twig/extensions": "~1.0",
         "sonata-project/exporter": "~1.0",
         "sonata-project/block-bundle": "~2.2,>=2.2.7",