add `version` helper method to `caveats` mini-DSL

As was already done (for convenience) for `title`.

References: #4921 .

This is somewhat contradictory with #4926, where an example was given using
`@cask.version` within the `caveats` mini-DSL.  But even if this PR is merged,
I think `CASK_LANGUAGE_REFERENCE.md` still needs a similar section,
explaining how to use the `@cask` instance variable.