Design, Synthesis and Antiviral Activity of Novel 4,5-Disubstituted 7-(β- d -Ribofuranosyl)pyrrolo[2,3-d][1,2,3]triazines and the Novel 3-Amino-5-methyl-1-(β- d -ribofuranosyl)- and 3-Amino-5-methyl-1-(2-deoxy-β- d -ribofuranosyl)-1,5-dihydro-1,4,5,6,7,8-hexaazaacenaphthylene as Analogues of Triciribine