Added graphviz to travis install line.
Added graphviz to travis install line.

laprej/ROSS-CXX,carothersc/ROSS,laprej/ROSS-CXX,JohnPJenkins/ROSS,JohnPJenkins/ROSS,carothersc/ROSS,JohnPJenkins/ROSS,laprej/ROSS-CXX,carothersc/ROSS