py-wand: updated to 0.6.7

Version 0.6.7
- Added :meth:`Image.image_add() <wand.image.Image.image_add>` method.
- Added :meth:`Image.image_get() <wand.image.Image.image_get>` method.
- Added :meth:`Image.image_remove() <wand.image.Image.image_remove>` method.
- Added :meth:`Image.image_set() <wand.image.Image.image_set>` method.
- Added :meth:`Image.image_swap() <wand.image.Image.image_swap>` method.
- Fixed sub-image extraction on read. [:issue:`532`]
- Fixed :attr:`~wand.image.BaseImage.background_color` attribute when image was not read.
- [DOC] Completed :doc:`Distortion <./guide/distortion>` guide. [:issue:`534`]
- [DOC] Added :doc:`Morphology <./guide/morphology>` guide.

(adam)