Camera rolling around Z axis

I write a simple script to control a camera with mouse like this:

void Update () {

	transform.Rotate (Vector3.right * Input.GetAxisRaw ("Mouse Y") * Time.deltaTime);
	transform.Rotate (Vector3.up * Input.GetAxisRaw ("Mouse X") * Time.deltaTime);
}

but the camera is rolling around the Z axis. I think i need to set the Z axis rotation to 0 , but i cant.

You want something like this:

    public float speedH = 2.0f;
public float speedV = 2.0f;

private float yaw = 180f;
private float pitch = 0.0f;

void Update () {
	yaw += speedH * Input.GetAxis("Mouse X");
	pitch -= speedV * Input.GetAxis("Mouse Y");

	transform.eulerAngles = new Vector3(pitch, yaw, 0.0f);
}