[FIX] .travis.yml: Latest Travis template

Author: pedrobaeza

.travis.yml

@@ -1,29 +1,15 @@
 language: python
 sudo: false
-cache:
-  apt: true
-  directories:
-    - $HOME/.cache/pip
+cache: pip
 
 python:
   - "3.5"
 
 addons:
   postgresql: "9.6"
   apt:
-    sources:
-      - pov-wkhtmltopdf
     packages:
       - expect-dev  # provides unbuffer utility
-      - python-lxml # because pip installation is slow
-      - python-simplejson
-      - python-serial
-      - python-yaml
-      - wkhtmltopdf  # only add if needed and check the before_install section below
-
-before_install:
-  - "export DISPLAY=:912.0"
-  - "sh -e /etc/init.d/xvfb start"
 
 env:
   global: