This is currently not possible with lein (bug/enhancement), but will be with 2.1.0. See #966[1] for details. As a workaround, any project dependent of Clojure 1.5.0 will utilize that for the repl. So when in a project using 1.5.0, the repl will be 1.5.0 too.

[1]: https://github.com/technomancy/leiningen/issues/966