Use Time.clone rather than Time.dup which preserves the class methods on old versions of Ruby.

I'm not sure of the performance implications of this, but if we `clone` rather than `dup` `Time`, we can fix #894. The Ruby docs state that `clone` copies the internal state whereas `dup` does not... but as we're dealing with a class, that shouldn't be an issue?

(Sanity check please @myronmarston @samphippen @soulcutter)