NOTE:This feature is only available on versions of Graphviz that are newer than mid-November 2003. In particular, it is not part of release 1.10.
NOTE:The font markups for bold, italic, underlining, subscript and superscript (<B>, <I>, <U>, <SUB> and <SUP>) are only available in versions after 14 October 2011, and the markup for strike-through (<S>) requires versions later than 15 September 2013. In addition, all of these markups are currently only available via the cairo and svg renderers. The horizontal and vertical rules (<HR> and <VR>) are only available in versions later than 8 July 2011.
In https://github.com/clue/graph/pull/96/files graphviz is added to .travis like
From the documentation on http://www.graphviz.org/content/node-shapes#html