setup.py: add dependency link for ruaumoko

Hello all,

At your lovely fresher's squash I asked @adamgreig if there was any projects he thought it would be worth pointing my eyes at since I'd like to get involved with CUSF this year. He suggested I look at the ol' predictor again and so here I am: :wave:.

I'll come along to the first meeting when it's announced on -team but since I have a spare afternoon to play with the predictor, I thought I'd try to get it up and running on my machine so I hope it's OK to throw some trivial build-related PRs at you.

As it stands, trying to git clone the repo and perform the standard
``pip install -r requirements.txt`` or ``python setup.py develop`` dance
will fail unless you have ruaumoko[1] installed. If it is not installed,
setuptools attempts to download it from PyPI which fails.

Add a ``dependency_links`` parameter to the call to ``setup`` which
specifies where to fetch the code for ruaumoko.

[1] https://github.com/cuspaceflight/ruaumoko/