neil dep upgrade should update unstable versions

[teodorlu]

It would be nice to use neil dep upgrade before there are any stable released versions of a library. Per 0.1.60, running neil dep upgrade when when the installed version is unstable and the there are more recent unstable versions does nothing.

I believe we should keep neil dep upgrade behavior unchanged when we're on a stable version, stable versions can only be upgraded to more recent stable versions.

The following situation is less straightforward:

1. user has installed lib 0.1-alpha.
2. both a more recent stable version 0.2 and an even more recent unstable version are available 0.3-alpha are available.

I have a weak preference for "when the current version is unstable, pick the latest version whatsoever".

[borkdude]

I think a stable version should be preferred no matter what, unless there is no version, then an unstable version is the best you can do. We can include additional flags to choose --unstable.

[teodorlu]

Closing, as I consider this to be fixed!