Use only iv column and not salt by default

[ribose-jeffreylau]

https://github.com/attr-encrypted/attr_encrypted#the-mode-option

Our dependency on attr_encrypted means this project should act on deprecations on modes involving salt.

Since we haven't released any official versions, I think it's safe to directly remove mentions of salt in the code, e.g., db/migrate/20180822000001_create_schema.rb, without risking breakage of other people's production code.

[ronaldtse]

Agreed!