add min-lein-version

I was working with someone at the Austin Clojure Meetup who was trying
to deploy a compojure template app to Heroku (actually, Dokku, which
uses the Heroku buildpacks).  He got tripped up by the template not
setting `min-lein-version`.  Since this is a starting point for many new
Clojure apps, maybe we should set it by default?