Ar preserve hashes

reopen of https://github.com/attr-encrypted/attr_encrypted/pull/117
I think this is introducing slightly different behaviour than we had before. Previously we only included actionpack if `ENV['ACTIVERECORD'] && ENV['ACTIVERECORD'] > '4.0'`. Now we are including it regardless of the activerecord version. I am unsure how this affects us and it may not even be an issue.