Update travis configuration

use sudo: false to enable container build
use cache: pip to cache pip packages

.travis.yml

@@ -1,8 +1,13 @@
+sudo: false
+cache: pip
+
 addons:
   apt:
     packages:
       - expect-dev  # provides unbuffer utility
       - python-lxml # because pip installation is slow
+      - cups
+      - libcups2-dev
 
 language: python
 python:
@@ -16,15 +21,10 @@ env:
 virtualenv:
   system_site_packages: true
 
-before_install:
-  - sudo apt-get update
-  - sudo apt-get install libcups2-dev cups
-
 install:
   - git clone https://github.com/OCA/maintainer-quality-tools.git $HOME/maintainer-quality-tools
   - export PATH=$HOME/maintainer-quality-tools/travis:$PATH
   - travis_install_nightly
-  - pip install coveralls flake8
   - pip install pycups==1.9.66
   - pip install PyPDF2==1.18
   - pip install requests