Add Swift version to Podspec

[thorstenstark]

We should explicitly set the supported Swift version in Podspec file to avoid possible errors when integrating into a project that uses a different Swift version.

[thorstenstark]

https://blog.cocoapods.org/CocoaPods-1.4.0/ -> "Note: The usage of the .swift-version file as well as the --swift-version parameter during lint is now deprecated."