Bump the leiningen version that the crate downloads.

Technomancy says that v2.0-previewX is production ready. The only reason leiningen's version hasn't been bumped to v2.0 is because of some dependency on clojars.org.

NOTE: If you want, I'll submit a patch that allows the user to a URL to pull the lein script from. This way the user can override the default.