Upgrade to Scalaz 7.10 and all that entails

I thought this was going to be "harder", so I suspect I'm missing something pretty serious.

/cc @etorreborre @markhibberd 
@markhibberd Added this as per your [suggestion](https://github.com/ambiata/ivory/pull/163#discussion_r15746051). My only concern is that if/when we pull this out into a library I don't think you would depend on scalaz just for this.