Add a future 1.5.0 Version constant

I'm not sure if this is something we want to do, but I think it makes it easier to plan PRs. I wanted to submit this and see if people had thoughts about it.

Since our git workflow includes not rebasing a branch once a PR has been submitted (rightly so!), this makes it possible to submit PRs during the 1.4.0 development cycle that target 1.5.0, without this all PRs have to target 2.0.0 and then fix-up their version checks once 1.4.0 is actually released since the new `Version` is not added until then.