Fix Travis

At some point Travis apparently decided to set the default of the `sudo`
option to `false`. But that would mean I had to whitelist every package,
which is not acceptable.

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

.travis.yml

@@ -1,3 +1,4 @@
+sudo: true
 language: python
 python:
     - "2.7"