Use Clojure 1.6.0-rc3

Signed-off-by: Ken Barber <ken@bob.sh>
Clojure 1.6.0-RC4

Bump clojure to 1.6.0-RC4 instead, now that it was released quickly behind
RC3.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #893 from kbarber/maint/master/clojure-1.6.0-rc3

Use Clojure 1.6.0-rc3