Prevent analyzer warning by using non-js base-type

The CLJS analyzer warns against using any of the symbols in js-base-type map
https://github.com/clojure/clojurescript/blob/a10080d7743e480f689d50e2558415e6b63e7632/src/clj/cljs/core.clj#L605-L621
Merge pull request #96 from Prismatic/function-base-type

Prevent analyzer warning by using non-js base-type