Renamed 'unstable' to 'beta'.

Our dev processes are pretty darn good now, and we have a lot more
eyes on code. I'd like to change `unstable` releases to `beta` releases.
(It's good enough for Google and Squad!)
Merge pull request #594 from pjf/beta_than_anything

Renamed 'unstable' to 'beta'.
Copy downloads before iterating over them, since they may be modified concurrently.

Without this fix I repeatably see 'System.InvalidOperationException: Collection was modified; enumeration operation may not execute.' exceptions while installing KarbonitePlus.
Merge pull request #601 from cswetenham/master

Copy downloads before iterating, since they may be modified concurrently
Merge pull request #600 from AlexanderDzhoganov/master

Fix the missing default instance bug
Fixes #602.
Merge branch 'release-v1.2'

* release-v1.2:
  Renamed 'unstable' to 'beta'.
Merge pull request #607 from RichardLake/#602

Fixes: Gui shows non-updateable mods as having a update #602
Relationship resolver now uses KSP version in resolution.

`LatestAvailableWithProvides` now requires a mandatory KSP version, so
we don't accidentally do this again.
CKAN.dll: LatestAvailable now requires a version number

This means we can never go looking up mods which may be the wrong
version for our install by accident. It may also fix some edge cases
like #602 where we may show the wrong versions being available.

Most of the changes in this commit are to the test cases, which now
require the mandatory argument. :)

Tested that this closes #610 with our NEAR test example.
Merge pull request #611 from pjf/610_relationship_resolver

Bugfix: Always use current KSP version when resolving relationships