Fix composer.lock warning => composer update --lock

The composer lock was out of date. This fixes the issue:

```
» composer install
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.

  - Installing symfony/yaml (v2.5.2)
    Downloading: 100%         

  - Installing symfony/finder (v2.5.2)
...
```