circe-spray 0.7.0

This release updates the circe version to 0.7.0 (and therefore transitively depends on Cats 0.9) and the Spray version to 1.3.4.

Spray 1.3.4 is not published for Scala 2.10, so updating means that we can no longer support 2.10 either. If anyone desperately needs this project published for 2.10, please open an issue and we can consider a special release depending on Spray 1.3.3.