Podspec trouble

I’m trying to install a Pod as per tutorial iOS Frameworks Tutorial (https://www.raywenderlich.com/126365/ios-frameworks-tutorial)). When I try to do ‘pod install’ it seems there’s an issue with the podspec file.
It returns:

[!] The ThreeRingControl pod failed to validate due to 1 error.
[!] The validator for Swift projects uses Swift 3.0 by default, if you are using a different version of swift you can use a .swift-version file to set the version for your Pod. For example to use Swift 2.3, run:
echo "2.3" > .swift-version:

Can someone please advise how to correct pod version?