Install graphviz in Phabricator container

Summary:
This will allow remarkup documents (tickets, wiki) to include graphviz graphs.
http://stackoverflow.com/questions/27526193/using-graphviz-package-in-phabricator-wiki

Test Plan:
`vagrant up`. Created a test wiki document with a simple graph in it.
Saw said graph rendered.

Reviewers: #developers

Subscribers: cmounce

Differential Revision: https://phabricator.paperg.com/D25