Update apt-get listing because graphviz update.

TravisCI is looking for http://security.ubuntu.com/ubuntu/pool/main/g/graphviz/graphviz_2.26.3-10ubuntu1.1_amd64.deb (which doesn't exist). 
It appears that graphviz was updated to graphviz_2.26.3-10ubuntu1.2_amd64.deb
Until the package index get update, let update for ourselves.