Speed up your travis-ci builds with container based infrastructure

Travis-ci just released container based infrastructure for open source repository (free users).

We tried it with our apps, and you can see we had some performance increase, even on database-intensive jobs:

• django-autocomplete-light normal build takes around 1 hour and 35 minutes while it takes rather 1 hour and 22 minutes with container based build](https://travis-ci.org/yourlabs/django-autocomplete-light/builds/45086517) so that’s around a 10% build speed increase, pretty cool right ? read on ;)
• django-cities-light normal build takes around 5 hours and 45 minutes, on travis-container it takes around 3 hours and 30 minutes !
• No notable performance increase to note on django-session-security.