Forcibly disable potemkin for CLJS

Before emitting potemkin/defrecord+, ensure that we aren't compiling for
ClojureScript. This may not be the ideal location for this check, but appears to
fix the problems described in Prismatic/schema#34
Add support for :pre/:post conditions in functions.
Merge pull request #45 from seancorfield/prepost

Add support for :pre/:post conditions in functions.
Merge pull request #44 from loganlinn/disable-cljs-potemkin

Forcibly disable potemkin for CLJS