Move setting of default version to "taskfile" package

internal/taskfile/taskfile.go

@@ -9,6 +9,7 @@ type Taskfile struct {
 // UnmarshalYAML implements yaml.Unmarshaler interface
 func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal(&tf.Tasks); err == nil {
+		tf.Version = "1"
 		return nil
 	}