Use clojure.core/read-string to drop 1.5+ dependency

In my opinion, since we're reading user-supplied values from the
command-line, that it is acceptable to use clojure.core/read-string over
edn/read-string. Doing this would remove our reliance on any particular
version of Clojure.

/cc @xsc: what is your opinion on this?

Tagging along with this pull is a shift downward of the minimum lein
version to 2.1.3