Support `language:generic` and `language:minimal` in Windows

Currently, Windows accepts language: shell – which has no default script, but doesn’t accept language: generic and language: minimal – which are the designated configurations without a default script.

This has been creating roadblocks for me (example, but generally, I overwrite unsuccessful attempts with forced pushes) if I wish to check things in all 3 OSes and need language: generic functionality in Linux.

Since I’m already getting a Windows VM for language: generic and language: minimal, VM type selection logic isn’t an issue here and it’s sufficient to patch travis-build. I don’t care if generic and minimal would yield the same VM image (there’s only one kind of Windows image anyway AFAICS).

The PR has just been merged. Many thanks!