Fix: PG::SyntaxError: Replace 'DISCARD ALL' query to similer operations.

* DISCARD` was implemented on PostgreSQL since v8.3
  However, Redshift is based on PostgreSQL 8.0.2

References
* [PostgreSQL DISCARD](https://www.postgresql.org/docs/current/static/sql-discard.html)
* [Amazon Redshift and PostgreSQL](http://docs.aws.amazon.com/redshift/latest/dg/c_redshift-and-postgres-sql.html)

lib/active_record/connection_adapters/redshift_adapter.rb

@@ -244,7 +244,10 @@ def reset!
         unless @connection.transaction_status == ::PG::PQTRANS_IDLE
           @connection.query 'ROLLBACK'
         end
-        @connection.query 'DISCARD ALL'
+        session_auth = 'DEFAULT'
+        @connection.query 'RESET ALL'
+        dealloc 'ALL'
+        @connection.query 'CLOSE ALL'
         configure_connection
       end