update gitignore with cover
update gitignore with cover

wattlebird/pystruct,pystruct/pystruct,d-mittal/pystruct,massmutual/pystruct,d-mittal/pystruct,amueller/pystruct,massmutual/pystruct,pystruct/pystruct,amueller/pystruct,wattlebird/pystruct