Support psr/cache v2

[jgivoni]

There doesn't seem to be any breaking changes affecting this library between versions 2 and 3 of psr/cache:
https://github.com/php-fig/cache/blob/master/CHANGELOG.md
(The difference is that in v3 "All methods have return types")
So if you're already using psr/cache v2 and cannot upgrade because of this change, this library won't install.
However, if this library already works with psr/cache v2, there's no good reason not to allow it.

• Verify and test this assumption.
• Update composer.json like this: