Add crystal: previous option

[Sija]

I'd like to suggest adding previous (or previous-minor) as the version option (following latest), allowing automatically selecting previous MINOR version for testing. That would help to catch any new additions in terms of BC.

[oprypin]

What's "BC"?

[Sija]

Backward Compatibility

[straight-shoota]

For backwards compatibility, wouldn't it be better to specify explicit versions that you want to ensure compatibility with? previous would just be a moving target, so it's not exactly clear which versions are considered to be backward compatible.

[Sija]

@straight-shoota That's exactly my point, the case at-hand is testing features that are working with Crystal latest onwards and checking whether the build still succeeds with the previous one, like in this PR: crystal-ameba/ameba#536

[straight-shoota]

But how is a floating target latest better than pinning a specific version?

[Sija]

It's not about better, but a specific use-case.