Issue 301 - Switch to new TravisCI infrastructure

By switching to the new insfrastructure we get:
* more isolation (thanks to containers)
* faster builds
* CPU resources are guaranteed
* caching available

caveat: As you can’t use sudo on the new container-based infrastructure,
you need to use the addons.apt.packages and addons.apt.sources plugins
to install packages and package sources

[1]
http://docs.travis-ci.com/user/migrating-from-legacy/?utm_source=legacy-notice&utm_medium=banner&utm_campaign=legacy-upgrade

.travis.yml

@@ -1,3 +1,5 @@
+sudo: false
+
 language: python
 
 python: