Adding graphviz to travis

Author: Jordan Hoff

.travis.yml

@@ -9,6 +9,10 @@ env:
     - COMPOSER_FLAGS="--prefer-lowest"
     - COMPOSER_FLAGS=""
 
+before_install:
+  - sudo apt-get update
+  - sudo apt-get install -y graphviz
+
 before_script:
   - travis_retry composer self-update
   - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-source