hotfix-iss36: remove cleanup_namespaces for temp fix

Removing the objectify.deannotate(cleanup_namespaces=True) attribute
from the XML serializer used by Presentation.save() provides a fix for
this error but is somewhat messy and has broad effects. A more
permanent solution will take more work and must be completed before
merging this into develop and master branches.

Conflicts:
	pptx/oxml.py
	pptx/presentation.py
	pptx/shapes.py
	test/test_presentation.py
	test/testdata.py
	test/testing.py
	tests/test_files/minimal_slide.xml