It's Heyward. The printed score I have of Porgy and Bess credits "DuBose Heyward" and so do these:

http://www.britannica.com/EBchecked/topic/264727/DuBose-Heyward

http://www.duboislc.org/ShadesOfBlack/DuBoseHeyward.html

http://www.ibdb.com/person.asp?id=5141

http://xroads.virginia.edu/~hyper/PORGY/porghome.html

http://en.wikipedia.org/wiki/DuBose_Heyward

http://books.google.co.uk/books?hl=en&id=fbTIs3QUqhkC&dq=dubose+heyward&printsec=frontcover&source=web&ots=iQgnC7iy70&sig=sgZA2uWx1py6l7b_PR0U7nQ_S9E&sa=X&oi=book_result&resnum=4&ct=result

Among the above links are photos showing his original book covers, which bear the name Heyward.