Fix the Form component deprecations for Symfony 3

[javiereguiluz]

If you try to create a new blog post (/en/admin/post/new):

If you try to edit a blog post (/en/admin/post/1/edit):

[bocharsky-bw]

Can you describe more how to reproduce it? You updated symfony/symfony to v3.0.0-BETA1?

[stof]

@bocharsky-bw yeah. In 2.8 you would get a deprecation warning instead of an exception

[javiereguiluz]

@bocharsky-bw the easiest way to update is to copy the composer.lock from here: https://github.com/benji07/symfony-demo/tree/symfony3 and execute a composer install