[ActiveRecord][PostgreSQL] Support for #next_sequence_value

The PostgreSQLAdapter does not currently define #next_sequence_value. If I have a User model, then User.respond_to?(:next_sequence_value) will return true, but calling it will raise a NoMethodError, since that method is delegated to the database adapter.

Implementing this seems to be fairly simple, pending any weirdness across different Postgres versions:

# ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
def next_sequence_value(sequence_name)
  execute("SELECT nextval(#{quote(sequence_name)})").first['nextval']

Am I missing anything that would make this change any more complicated? As is, this works locally for me (albeit only tested against the one version of Postgres I am running).