update clj-time to 0.6.0

Ring-core is using very old version of `clj-time`. Many web libs depend on the newer version, but the ring-core version keeps popping up (for example with `lein ring uberwar`, which adds it's ring-deps on the fly as top-most dependencies) => `clj-time` version needs to be manually set in the end projects despite not needing it directly. 

Example: https://github.com/metosin/compojure-api-examples/blob/master/project.clj#L4