Should keep the primary key column order

Fixes CI failure caused by rails#27961.

https://travis-ci.org/rails/rails/jobs/201472146#L2390-L2409

Author: kamipo

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -441,6 +441,7 @@ def primary_keys(table_name) # :nodoc:
              WHERE constraint_type = 'PRIMARY KEY'
                AND kcu.table_name = #{quote(name.identifier)}
                AND kcu.table_schema = #{name.schema ? quote(name.schema) : "ANY (current_schemas(false))"}
+             ORDER BY kcu.ordinal_position
           SQL
         end