Travis: add apt-get update

.travis.yml

@@ -17,6 +17,7 @@ install:
     - pip install --quiet --use-mirrors tox
     # install python 3.4 from PPA since Travis does not have python 3.4 yet
     - sudo apt-add-repository -y ppa:fkrull/deadsnakes
+    - sudo apt-get update
     - sudo apt-get install python3.4
 script:
     - tox