Fix travis.yml

.travis.yml

@@ -1,15 +1,16 @@
 language: python
-sudo: false
-cache:
-  apt: true
-  directories:
-    - $HOME/.cache/pip
 
 python:
   - "2.7"
 
+sudo: false
+cache: pip
+
 addons:
+  postgresql: "9.6"
   apt:
+    sources:
+      - pov-wkhtmltopdf
     packages:
       - expect-dev  # provides unbuffer utility
       - python-lxml  # because pip installation is slow
@@ -18,10 +19,7 @@ addons:
       - python-yaml
       - cups
       - libcups2-dev
-
-before_install:
-  - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
+      - wkhtmltopdf
 
 env:
   global: