Make work with Julia 1.5, except for chordal embeddings

[jnshi]

I made these changes to make the code work with Julia 1.5, since I had difficulty installing older versions of Julia. They're not great, but I tested against the notebooks and made sure they worked and the results were the same or close enough.

This commit only contains changes in src. I also have the updates to the notebooks, but didn't want to commit them because (1) I never figured out how to make using Polyopt work and so I used include("Polyopt.jl") instead (2) changes to notebooks can't be diffed.

The exception is the chordal embeddings: I wasn't able to figure out how the old code with cholfact() worked and so couldn't update to the new cholesky() interface. Also there's a warning about not being able to import Base.sub, which I didn't know what that was supposed to do, so I wasn't able to fix that.

Possibly you'll want to keep both versions of the code, in case someone is still using the Julia 0.6 build?

[joachimdahl]

We are transferring ownership of this project. It now resides here:
https://github.com/ahmadreza-marandi/Polyopt.jl

Can you please resubmit you changes to that site, and write an acknowledgment here when you're done. Then we will delete the project from the MOSEK folder.

[jnshi]

We are transferring ownership of this project. It now resides here:
https://github.com/ahmadreza-marandi/Polyopt.jl

Can you please resubmit you changes to that site, and write an acknowledgment here when you're done. Then we will delete the project from the MOSEK folder.

done.