Add retry? method to Hash to be used in shrinkify
Add retry? method to Hash to be used in shrinkify

`Rantly::Property#shrinkify` calls `retry?` which is defined for
all classes in `shrinks.rb` except `Hash`. Consecuently it raises an
`NoMethodError` when being called over a Hash.

Fixes https://github.com/rantly-rb/rantly/issues/39

Ana06/rantly,abargnesi/rantly