[Installer] Make finding minimum object version more efficient

lib/cocoapods/installer.rb

@@ -445,8 +445,8 @@ module Pod
     def prepare_pods_project
       UI.message '- Creating Pods project' do
         object_version = aggregate_targets.map(&:user_project_path).compact.map do |path|
-          Xcodeproj::Project.open(path)
-        end.map(&:object_version).map(&:to_i).sort.first
+          Xcodeproj::Project.open(path).object_version.to_i
+        end.min
 
         if object_version
           @pods_project = Pod::Project.new(sandbox.project_path, false, object_version)