Unity3D Rotate GameObject with mouse

using UnityEngine;
using System.Collections;

public class RotateWithMouse : MonoBehaviour {
 
 public float sensitivityX = 15.0f;
 public float sensitivityY = 15.0f;
 private Transform cameraTm;
 
 public bool down = false;

 // Use this for initialization
 void Start ()
 {
  cameraTm = Camera.mainCamera.transform;
 }
 
 // Update is called once per frame
 void Update () {
  if( Input.GetMouseButtonDown( 0 ) )
   down = true;
  else if( Input.GetMouseButtonUp( 0 ) )
   down = false;
  
  if( down )
  {
   float rotationX = Input.GetAxis("Mouse X") * sensitivityX;
   float rotationY = Input.GetAxis("Mouse Y") * sensitivityY;
   transform.RotateAroundLocal( cameraTm.up, -Mathf.Deg2Rad * rotationX );
   transform.RotateAroundLocal( cameraTm.right, Mathf.Deg2Rad * rotationY );
  }
 }
}