Bump version of clj-time to play nice with Ring usage of newer API

Earlier this month, Ring updated its dependency on clj-time to 0.6.0, and switched to using new API functions, particularly `in-seconds` for cookies. https://github.com/ring-clojure/ring/commit/7d618a9d4e0ff0cf0368cd159f9b786e904332d2

Optimus depends on clj-time 0.5.1, which seems to be what gets resolved and used, so Ring's reference to `in-seconds` fails with a fatal IllegalAccessException.

The PR bumps Optimus's dependency on clj-time to 0.6.0 too. (It might even be safe to use 0.7.0 which removes deprecated functions, as Optimus doesn't seem to use any of them.) All unit tests pass and I can't find any likely breakages with the change.

A workaround for anyone else having this problem is to just depend on clj-time 0.6.0 explicitly in your own project.clj.