Auto merge of #3841 - matklad:encourage-explicit-version, r=alexcrichton

Encourage tools writers to explicitly pin metadata version

We do support versioning of metadata, but let's encourage tool's writers to actually use it.

They might not realize that this flag exists at all, or they can be too lazy (like myself :( ) to use it.

We can also make this flag mandatory, but I think that's a little bit to far.