[FIX] Update .travis.yml.

.travis.yml

@@ -1,7 +1,10 @@
 sudo: false
-cache: pip
+cache:
+  directories:
+    - $HOME/.cache/pip
 
 addons:
+  postgresql: "9.6"
   apt:
     packages:
       - expect-dev  # provides unbuffer utility
@@ -10,17 +13,14 @@ addons:
 language: python
 
 python:
-  - "2.7"
+  - "2.7.13"
 
 env:
   - VERSION="6.1"
 
-virtualenv:
-  system_site_packages: true
-
 install:
   - pip install python-magic
-  - git clone https://github.com/OCA/maintainer-quality-tools.git ${HOME}/maintainer-quality-tools
+  - git  --depth=1 clone https://github.com/OCA/maintainer-quality-tools.git ${HOME}/maintainer-quality-tools
   - export PATH=${HOME}/maintainer-quality-tools/travis:${PATH}
   - pip install --update pyth
   - travis_install_nightly ${VERSION}