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
Merge pull request #2170 from lucasgranberg/patch-1

Update composer.json