0.5.5 violates PVP; re-release as 0.6

Hi @mchav , I saw your announcement of 0.5.5.0, thanks for all the work put in here! I checked the changes (https://github.com/haskell-hvr/cassava/compare/v0.5.4.1...v0.5.5.0) and saw that the `EncodeOptions` got a new field. Because an exported symbol changed its type, a major version bump is warranted by the Haskell Package Versioning Policy (https://pvp.haskell.org). This is so that packages using `cassava < 0.6` do not break (as long as they follow the PVP hygiene to import everything qualified). I suggest you re-release 0.5.5.0 as 0.6 and deprecate 0.5.5.0 on Hackage (https://hackage.haskell.org/package/cassava/maintain): 1. Mark it as deprecated here: https://hackage.haskell.org/package/cassava/preferred/edit 2. Revise it to `base < 0` so that the cabal constraint solver cannot select this version: https://hackage.haskell.org/package/cassava-0.5.5.0/cassava.cabal/edit