Unquoted scalars exception with Symfony 3.1

Hi,

While installing the bundle in my Symfony 3.1 app I get the following error.

```
[Symfony\Component\Yaml\Exception\ParseException]
    The reserved indicator "@" cannot start a plain scalar; you need to quote the scalar at line 9 (near "- @anca_rebeca_full_calendar.service.serializer").
```

The fix is very simple, I'll make a PR with the fix.
