Completions do not work for packages that have only pre-release versions available

[tintoy]

This is due to a bug in the NuGet v3 protocol client that fails to correctly inform the NuGet AutoComplete API end-point that pre-release packages should be included (see NuGet/Home#7540 for details).

Transferred from #44.

CC: @SimantoR

[tintoy]

NuGet folks are working on this; I imagine it won't be long before a fix is available. In order to upgrade, though, I'll have to drop support for NuGet v2 feeds (so only v3 feeds will be supported) because latest NuGet client libraries no longet support v3. This is a breaking change, but v3 feeds are sufficiently ubiquitous that I think we're OK to do that.

[tintoy]

NuGet issue has been resolved; will upgrade packages and see if it works now :)