I think it's better to test against the currently available released version and not the original versions of the set.

One way you could do this is by having this be 1.4, 1.5 and 1.6 and tweaking the pip install line so that it's `Django<$DJANGO_VERSION`. It's a little odd looking, but then it automatically always picks up the right latest version without someone having to update the .travis.yml file all the time.