Drop Julia v0.x as development branch dependency

[jrevels]

As discussed offline as well as in https://github.com/ModiaSim/ModiaMath.jl/pull/10, we should not attempt to maintain a codebase against several language versions simultaneously. As I said in that issue:

A pretty standard (and much less painful way, IMO) to continue maintaining support for older language major versions is to cut separate release branches for the different Modia major versions you wish to continue to support, and then backport any changes that need backporting. That way, you can support all the language version you want, but don't need to maintain a single codebase that supports multiple versions of the same language. It can be tricky if you have a lot of backports every update, but it's usually not too bad - even for larger packages/ecosystems (like JuMP). Dealing with fewer edge-cases makes it easier to iterate.

There are apparently a few complications to actually doing this (quoting @MartinOtter):

The trouble is that Modia and Modia3D depend on ModiaMath and neither Modia nor Modia3D have been yet fully converted to Julia 0.7/1.0 and there might be still issues in ModiaMath that need to be fixed before Modia/Modia3D have been fully converted. Furthermore, development is continuing in parallel. Therefore, the strategy is to have one version of Modia/ModiaMath/Modia3D supporting Julia 0.6, 0.7, 1.0 and making releases of these versions (and accepting that the code is not nice at some places). After these releases, the support for Julia 0.6 and 0.7 is dropped and the development of all three packages continues only on the basis of Julia 1.0.

The most critical issue preventing currently to switch to Julia 1.0 is a bug in Julia. Once this bug is fixed, switching to Julia 1.0 should be quick (a few days).

The referenced bug: https://github.com/JuliaLang/julia/issues/28917