Add graphviz installation instruction

brexis · brexis · commit 68fa5c2f3e1e · 2017-12-01T01:06:30.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,10 +1,11 @@
 language: php
 
 php:
-  - 5.4
-  - 5.6
   - 7.0
   - 7.1
+before_install:
+  - wget http://204.178.9.49/pub/graphviz/stable/ubuntu/ub13.10/x86_64/graphviz_2.38.0-1~saucy_amd64.deb
+  - sudo dpkg -i graphviz_2.38.0-1~saucy_amd64.deb
 
 before_script:
   - composer self-update
