CocoaPods Podfile post_install script for adding swift_version property to targets or pod_project. You can use this post_install script if you are working with pods that not compatible with Swift 3.0.

xcode8-swift2.3-cocapods.podfile.rb

# set true if all pods in the pod_project needs to be flaggged as swift 2.3
legacy_swift_pods_project = true 
# Array for pods that will be flagged as swift version 2.3
# if legacy_swift_pods_project is true, you don't need to add any item in array.
legacy_swift_pods = ['Alamofire'] 

post_install do |installer|
    if legacy_swift_pods_project
        installer.pods_project.build_configurations.each do |config|
            config.build_settings['SWIFT_VERSION'] = '2.3'
        end
    end
    
    installer.pods_project.targets.each do |target|
        if legacy_swift_pods.include? target.name
            target.build_configurations.each do |config|
                config.build_settings['SWIFT_VERSION'] = '2.3'
            end
        end
    end
end