Install graphviz package for Travis CI builds
Install graphviz package for Travis CI builds

This avoids tons of messages about dot being not found when running
Doxygen and, more generally speaking, should test the build in more
realistic environment.

vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp