Don't use enable_extension under Sequel, which doesn't support it

[Empact]

As noted here:

It's trivial to use run "CREATE EXTENSION citext" in your migration if you want to add the extension. https://github.com/jeremyevans/sequel/pull/819#issuecomment-43821533

Note enable_extension uses "IF NOT EXISTS", as per: https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html#method-i-enable_extension

[janko]

Thanks for the fix 👍🏻