Install the graphviz apt-package and graphviz lib in travis conf

.travis.yml

@@ -9,9 +9,14 @@ python:
   - "3.6"
   - "3.7"
   - "3.8"
+addons:
+  apt:
+    packages:
+      - graphviz
 before_install:
   - pip install -U pip>=8.0.2
   - pip install pytest
+  - pip install graphviz
 install: pip install .
 script: pytest -v tests/test_pipdeptree.py
 sudo: false