Fix execution sequence of configuration methods when Padrino::Application is inherited

Currently, we couldn't use the register keyword inside the
configure_apps block, so this fixes it.

@example
  NewPlugin = Module.new
  Padrino.configure_apps { register NewPlugin }
  class App < Padrino::Application; end
Merge pull request #1842 from padrino/fix-execution-sequence

Fix execution sequence of configuration methods when Padrino::Application is inherited