Add sequel_pg gem.

http://sequel.jeremyevans.net/rdoc/files/doc/postgresql_rdoc.html says:

"When the postgres adapter is used with the pg driver, Sequel
automatically checks for sequel_pg, and loads it if it is available.
sequel_pg is a C extension that optimizes the fetching of rows,
generally resulting in a 2-6x speedup. It is highly recommended to
install sequel_pg if you are using the postgres adapter with pg.

sequel_pg has additional optimizations when using the Dataset map,
to_hash, to_hash_groups, select_hash, select_hash_groups, select_map,
and select_order_map methods, which avoids creating intermediate hashes
and can add further speedups.

In addition to optimization, sequel_pg also adds streaming support if
used on PostgreSQL 9.2. Streaming support is similar to using a cursor,
but it is faster and more transparent."