Provide a default version for leiningen

For odd cases where leiningen is called without the wrapper shell script, any occurence of `:min-lein-version` will throw an exception since `leiningen-version` will return nil.

This fixes jenkinsci/leiningen-plugin#1 for instance.
I see two other ways to fix this:

- have the version be propagated from project.clj directly, through substitution
- let leiningen-version return a string that always passes or always fails (not sure which is best) the `:min-lein-version` requirement