Change deployment target from 9.0 to 8.0
Change deployment target from 9.0 to 8.0

Previously, our dependencies required us to update the deployment target to 9.0. Since we have removed the dependencies, however, we can now lower the deployment target to 8.0 for greater compatibility.

watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk