auth: default admin/admin credentials on first install + bump 1.0.4

SeedAdmin now uses 'admin'/'admin' as defaults when environment
variables are not set, so the service works out-of-the-box after
first install.

cmd/server/main.go

@@ -20,7 +20,7 @@ import (
 	"github.com/syncserver/internal/syncengine"
 )
 
-var version = "1.0.3"
+var version = "1.0.4"
 
 func main() {
 	cfgPath := flag.String("config", "", "Path to config.yaml")