Fixed dev dependencies in composer.json

Since jms/translation-bundle is only working with sf 2.2 from
version 1.1.x-dev, I updated the composer.json file to add the
@dev flag to the translation bundle version definition.

Now the --dev requirements can be isntalled again.
Merge pull request #1298 from tPl0ch/hotfix/composer-dev-requirements

Fixed dev dependencies in composer.json