Need gtk2, not 3

hadley · hadley · commit b5609e5d32ac · 2018-07-06T09:14:14.000-05:00
diff --git a/.travis.yml b/.travis.yml
@@ -8,6 +8,6 @@ addons:
   apt:
     packages:
       - libxml2-dev
-      - libgtk-3-dev
+      - libgtk2.0-dev
       - ggobi
 
