Revert "Migrate CI to container-based infrastructure"

This reverts commit bd164b2.

Apparently the mongo service is started with a sudo, which is
not supported by the Travis-docker intrastructure. Took me a while
to realize that.

.travis.yml

@@ -1,6 +1,4 @@
 language: python
-sudo: false
-cache: pip
 python: 2.7
 env:
   - TOX_ENV=py26