config/environments/development.rb:1:in `<top (required)>': undefined method `configure'

Rails 4 application is throwing the errors during the startup with
development. I use mongodb for DB.

flex/config/environments/development.rb:1:in `<top (required)>':
undefined method `configure' for #<Flex::Application:0x007f49ec793870>

Ruby version ruby 2.0.0dev (2013-02-08 trunk 39161)

pls help on this

Is this a new Rails 4 app or an upgrade?

Did the problem appear when you changed something? If so what?