Added dependency doctrine/migrations

According to https://packagist.org/packages/doctrine/doctrine-migrations-bundle, doctrine-migrations-bundle requires doctrine/migrations besides symfony framework. If you follow documentation as is, you'll may get this error:
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for doctrine/doctrine-migrations-bundle dev-master -> satisfiable by doctrine/doctrine-migrations-bundle[dev-master].
    - doctrine/doctrine-migrations-bundle dev-master requires doctrine/migrations ~1.0@dev -> no matching package found.
  Problem 2
    - doctrine/doctrine-migrations-bundle dev-master requires doctrine/migrations ~1.0@dev -> no matching package found.
    - symfony/framework-standard-edition 2.3.x-dev requires doctrine/doctrine-migrations-bundle dev-master -> satisfiable by doctrine/doctrine-migrations-bundle[dev-master].
    - Installation request for symfony/framework-standard-edition 2.3.x-dev -> satisfiable by symfony/framework-standard-edition[2.3.x-dev].

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> for more details.

Read <http://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.