Switch 2.6 dependency to stable version

symfony/symfony dependency needs to be stabilized on 2.6 because right now a fresh stable install fetches 2.6.x-dev.

```
$ composer create-project symfony/framework-standard-edition src --prefer-dist
Installing symfony/framework-standard-edition (v2.6.0)
  - Installing symfony/framework-standard-edition (v2.6.0)

...

  - Installing symfony/symfony (2.6.x-dev 5b6a95c)
    Downloading: 100%
```