Fix installing haystack from github at specific commit using setuptools

I just opened this issue to track the problem with installing haystack from github at a specific commit. This was thought to be fixed in #489 but there still seems to be an issue with that as [described at the end of the PR](https://github.com/tangentlabs/django-oscar/pull/489#commitcomment-2508241).

I can reproduce the issue locally from a fresh checkout. The setup that works with ``make sandbox`` (meaning ``python setup.py develop`` is:
```
      dependency_links=['https://github.com/toastdriven/django-haystack/tarball/0e95d8696f8ba770f9c60152136aba32f5591fd6#egg=django-haystack-2.0.0-beta'],
```

However, I haven't tested whether this works with ``python setup.py install`` as well. We had issues with installing from a tarball like this before so I am not sure if it will work with the above solution or not. This needs some additional testing before putting it up as a PR.