`post_install` hook to set the Swift version of pods (add to the end of your project's podfile)

set_swift_version.post_install.ruby

post_install do |installer|
    print "Setting the default SWIFT_VERSION to 4.0\n"
    installer.pods_project.build_configurations.each do |config|
        config.build_settings['SWIFT_VERSION'] = '4.0'
    end

    installer.pods_project.targets.each do |target|
        if ['SomeTarget-iOS', 'SomeTarget-watchOS'].include? "#{target}"
            print "Setting #{target}'s SWIFT_VERSION to 3.0\n"
            target.build_configurations.each do |config|
                config.build_settings['SWIFT_VERSION'] = '3.0'
            end
        else
            print "Setting #{target}'s SWIFT_VERSION to Undefined (Xcode will automatically resolve)\n"
            target.build_configurations.each do |config|
                config.build_settings.delete('SWIFT_VERSION')
            end
        end
    end
end