Acceptance tests use asset pipeline

Update the acceptance tests to use Django-pipeline.

This required an update to our fork of django-pipeline that hasn't been merged yet: https://github.com/edx/django-pipeline/pull/1

Since our requirements.txt doesn't specify a commit in the django-pipeline repo, I think it makes sense to merge this PR first, then merge the django-pipeline PR.

@cpennington @jarv please review