Replace calls to DialogHelper with calls to QuestionHelper

DialogHelper was deprecated from symfony 2.5. [Ref: http://symfony.com/doc/current/components/console/helpers/dialoghelper.html]
SonataAdminBundle throws error while generating admin for entities from console due to the absence of DialogHelper. 
QuestionHelper is the new Helper to incorporate in lieu of DialogHelper. Fabien Potencier made some replacements in sensiolabs/SensioGeneratorBundle. [Ref: https://github.com/sensiolabs/SensioGeneratorBundle/commit/c4acbf3389121927f13a5adf615f00b8e5d8868e]

This patch will make GenerateAdminCommand compatible with new versions ( =>2.5) of symfony.