Series/6.0.x

Hi,

I've made EncodeJson and DecodeJson invariant because variance kinda messes up the codecs in our project. :) I see that you did pretty much the same in the master / 6.1-M2 branch but I can't use that yet because we use scalaz 7.0.x (and a couple of libraries that depend on that version).

Do you think it's feasible to include this into the 6.0.x series, or is there some kind of problematic side issue I've overlooked?

Thanks in advance