build against Symfony 2.6 instead of Symfony 3.0

After the `2.6` branch has been created on the Symfony repository, the
`master` branch has become the development branch for `3.0`. However,
builds should still happen for `2.6@dev` instead of `3.0@dev` for now.
Merge pull request #904 from xabbuh/symfony-2.6-branch-build

build against Symfony 2.6 instead of Symfony 3.0