Ensure rake tasks that don't need a DB connection can be run without one

( cherry picked from commit e6f3fe5d upstream )

When using ActiveRecord::Base.connection.active? without a DB
connection, we get a "PG::ConnectionBad: could not connect to server"
error because ActiveRecord::Base.connection doesn't exist. By using
ActiveRecord::Base.connected? we ensure we don't get this error if
the connection is missing, which is the all point of the
Gitlab::CurrentSettings#connect_to_db? method!

lib/gitlab/current_settings.rb

@@ -38,7 +38,7 @@ module Gitlab
                  true
                end
 
-      use_db && ActiveRecord::Base.connection.active? &&
+      use_db && ActiveRecord::Base.connected? &&
                 !ActiveRecord::Migrator.needs_migration? &&
                 ActiveRecord::Base.connection.table_exists?('application_settings')