Wednesday small patch

This is a small bugfix patch that fixes the following issues:

• Costs: It's now possible to perform free actions (like exploiting an asteroid/geoharvesting a planet) even when your cash balance is negative
• Skill Download: the technology could keep making an additional resource in some case when it shouldn't do that (after undoing a tech change or other change that added a product to a planet)
• Undo: Structure and technology HUD tabs fixes to prevent performing actions that are no longer available after undo.

This patch brings the version numbers up to (b686) on Windows and (b687) on Mac.