Fix assets installation
Fix assets installation

The `assetic:dump` command does not exist in Symfony 3. The `assetic` namespace has been replaced by `assets`. The new command to install assets is `assets:install`. 

This PR overrides the `deploy:assetic:dump` command so it works in Symfony 3. Ideally, the `deploy:assetic:dump` command would be renamed to something like `deploy:assets:install` in the main deploy task. However, this would mean overriding the entire list of tasks.
robfrawley/deployer,deployphp/deployer,jverdeyen/deployer,cjsewell/deployer,prosalov/deployer,oanhnn/deployer,deployphp/deployer,deployphp/deployer,torinaki/deployer,TheRatG/deployer,robfrawley/deployer,mbrodala/deployer,prosalov/deployer