Bump to require jvm 1.6
Bump to require jvm 1.6

This has been the requirement in Clojure itself since 2013 (clojure
1.6.0):
https://github.com/clojure/clojure/commit/26cfee0d739d091a4d87890b3effe3850728ce47

bbatsov/reply,bbatsov/reply,trptcolin/reply,trptcolin/reply