Define `update` only for previous Clojure(Script) releases
Fixup handling of Clojurescript case

* Macro correctly detects ClojureScript case
* Compare to the appropriate version
* Rename macro to not contain specific version
Add entry to changelog
Rename macro to indicate internal nature, add docstring
Merge pull request #78 from crisptrutski/use-core-update

Define `update` only for previous Clojure(Script) releases