[AC-5457] adjust .travis.yml to install django-accelerator:
[AC-5457] adjust .travis.yml to install django-accelerator:

- infer django-accelerator revision as global env var.
- pass that revision as ARG parameter on build.

masschallenge/impact-api,masschallenge/impact-api,masschallenge/impact-api,masschallenge/impact-api