Skip AddColumn check on Postgres 11 and up

[jsteiner]

As of Postgres 11, it is safe to add new column with a default value.

I think we could add a conditional here and skip this check if the database is Postgres 11 or higher:

https://github.com/LendingHome/zero_downtime_migrations/blob/master/lib/zero_downtime_migrations/validation/add_column.rb