Travis: Declare that we don't use sudo

This will apparently allow us to build on Travis's new container-based build
system, which is supposed to lead to faster builds. The catch is that we
don't get root access, but we don't need it anyway.

http://docs.travis-ci.com/user/migrating-from-legacy/

.travis.yml

@@ -9,3 +9,4 @@ python:
 install: pip install -e .
 before_script: pokedex setup -v
 script: py.test
+sudo: false