Update clj-time to 0.7.0

I was getting `java.lang.IllegalAccessError: in-seconds does not exist, compiling:(ring/middleware/cookies.clj:1:1)`. 

Turns out `ring-core`'s dependencies still use `clj-time "0.6.0"` which had `in-secs`, but it's source code uses `0.7.0`'s `in-seconds`.