[Validator] do not assume specific of targets when building file_accessors to test against

lib/cocoapods/validator.rb

@@ -208,7 +208,7 @@ module Pod
       installer = Installer.new(sandbox, podfile)
       installer.install!
 
-      file_accessors = installer.aggregate_targets.first.pod_targets.first.file_accessors
+      file_accessors = installer.aggregate_targets.first.pod_targets.map{ |target| target.file_accessors }.flatten
       @file_accessor = file_accessors.find { |accessor| accessor.spec == spec }
       config.silent
     end