Update apt sources in Travis CI builds.

.travis.yml

@@ -3,6 +3,7 @@
 # It is used automatically for the repositories on Github if it's found in the
 # root directory of the project.
 language: cpp
+sudo: required
 
 compiler: gcc
 
@@ -19,6 +20,7 @@ notifications:
         on_failure: change
 
 before_install:
+    - sudo apt-get update
     - sudo apt-get install -y libcppunit-dev
 
 env: