core: use before_script instead of before_install (Travis)

.travis.yml

@@ -1,8 +1,8 @@
 language: c
 
-before_install:
-    - sudo apt-get update -qq
+before_script:
     - echo 'APT::Install-Recommends "false"' | sudo tee -a /etc/apt/apt.conf
+    - sudo apt-get update -qq
     - sudo apt-get -y install devscripts equivs python-pip libenchant-dev
     - sudo mk-build-deps -i debian/control
     - sudo pip install msgcheck