Travis: Switch apt install from graphviz to libgraphviz-dev
Travis: Switch apt install from graphviz to libgraphviz-dev

jmuhlich/pysb,LoLab-VU/pysb,alubbock/pysb,johnbachman/pysb,LoLab-VU/pysb,pysb/pysb,johnbachman/pysb