fixed convos-backend startup command

for supervisord (and thus Dockerfile)

without this fix, 

```bash
command=/convos/vendor/bin/carton exec script/convos backend
```

returns

```
Usage: convos backend {start|stop|reload|restart|foreground|status|get_init_file}
```
Merge pull request #231 from LouisKottmann/patch-1

fixed convos-backend startup command