ci: Switch to Wily for packages too old in Trusty

Vivid is EOL soon, so move on now.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.travis.yml

@@ -25,7 +25,7 @@ before_install:
 
 install:
   # Install additional packages
-  - sudo apt-add-repository 'deb http://archive.ubuntu.com/ubuntu vivid main'
+  - sudo apt-add-repository 'deb http://archive.ubuntu.com/ubuntu wily main'
   - sudo apt-get update -qq
   - sudo apt-get install -qq python-mako gcc-arm-linux-gnueabihf make
   # Install kernel build environment