node.py: fixed tf.tranformations AttributeError

On some setups, node.py would fail with the following error:
Traceback (most recent call last):
  File "./node.py", line 98, in <module>
    q = tf.transformations.quaternion_from_euler(roll,pitch,yaw)
AttributeError: 'module' object has no attribute 'transformations'

This commit fixes that.