.NET core versions

@joshua-anderson @akoeplinger
Every time I try to install dotnet core, it just fails with any versions now, is there a list of version which is supported? It’d be good if we could have same as golang’s gimme and be able to install including previews, this is killing me.

As of now I’m trying to install and I’m getting:

2.2.104 is either an invalid version of dotnet or unsupported on this operating system
on all 3 OSes, I tried copying from travis’ documentation, no luck. And how do I test with preview versions?

Also, if I do something like:

dotnet:
  - 3.0.100-preview3-010431
  - 2.2.104
  - 2.2.103
  - 2.2.102

it doesn’t work for some reason, while looking at build, linux is detected but no windows and osx. How do I test multiple .NET versions on multiple OSes? (with build stages so I can do build and test parallely)?