Compatibility to Capistrano 2.14.1

My ubuntu system always wants to install `capistrano-2.14.1` and I always must uninstall it to get capifony working again, so I thought I have a look which changes are required to make it work with the newer version. Unfortunately I had to rename a task, because `load` as a taskname leads to an error. In this PR I renamed `doctrine:fixtures:load` to `doctrine:load_fixtures`, because it was the only `fixtures`-task anyway. I think the (new) name of the task, or even this PR as a whole is open for discussion now :smile: 

- 2.13.5 still works
- Tests passes (what also means, that there were no tests for this task :relaxed:)