Fix check for postgres database

[corylown]

We'd like this condition to pass when the postgres adapter is being used.

3.3.0 :001 > ActiveRecord::Base.connection == ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
 => false 

3.3.0 :002 > ActiveRecord::Base.connection.instance_of?(ActiveRecord::ConnectionAdapters::PostgreSQLAdapter)
 => true