Bump up to iron 0.6 (>= 0.5, < 0.7) (#65)
Bump up to iron 0.6 (>= 0.5, < 0.7) (#65)

* Bump up to iron 0.6 (Similar to https://github.com/iron/persistent/pull/64)

* Change its version to 0.4.0

iron/iron