Added graphviz dependency

.travis.yml

@@ -10,6 +10,7 @@ addons:
     - texlive-fonts-recommended
     - texlive-latex-extra
     - latex-xcolor
+    - graphviz
 
 env:
   global:

graph-lib/README.md

@@ -6,8 +6,8 @@ How to build this project
 
 To install the build dependencies of this project on a clean Ubuntu 14.04
 machine, first install a recent [snapshot version of
- racket](https://pre.racket-lang.org/installers/). Then run the following
- commands:
+ racket](https://pre.racket-lang.org/installers/). You will also need
+to install `graphviz`. Then run the following commands:
 
     cd graph-lib/
     sudo apt-get install git