graphviz isn't necessary

Author: ashnazg

.travis.yml

@@ -9,7 +9,6 @@ script:
 
 before_script:
  - sudo apt-get -qq update > /dev/null
- - sudo apt-get -qq install graphviz > /dev/null
  - phpenv rehash > /dev/null
  - composer selfupdate --quiet
  - composer install --dev