Use "recur" special form instead of simple recursion

"drone-do-for" had a tail position recursive call. This commit
replaces the tail call with the "recur" special form and adjusts
the argument list as required by recur.
See http://clojure.org/special_forms#recur
Merge pull request #2 from bruceadams/use-recur

Use "recur" special form instead of simple recursion