update the example of generated postgresql config
get rid of outdated "`server` with Different Backends" section

* I don't think people are running Rails under mongrel today
* it's better not to sudo gem install
* basically all we need to do is just bundle the server. No need to give its name as a parameter
* I could write ^^^, but I realized that it's not really about "command line"
Ruby 1.8 support had gone

instance_variable_names remains for internal use, but it's not that useful for the users anymore