def check_app_requirement(specs)
      missing = []
      specs.each do |spec|
        if wagon = wagon_class(spec)
          unless wagon.app_requirement.satisfied_by?(Wagons.app_version)
            missing << "#{spec} requires application version #{wagon.app_requirement}"
          end
        end
      end

      missing
    end
def check_app_requirement(specs)
      missing = []
      specs.each do |spec|
        if wagon = wagon_class(spec)
          unless wagon.app_requirement.satisfied_by?(Wagons.app_version)
            missing << "#{spec} requires application version #{wagon.app_requirement}"
          end
        end
      end

      missing
    end
Check if the app requirement of the given gem specifications
 are met by the current app version.
 Returns nil if everything is fine or a array with error messages.