The default Swift version leaves 4.0. Because it avoids build errors
with Carthage on each Xcode version from the source compatibility
between Swift 4.0, 4.2 and 5.0.
With regard to CocoaPods, the pod spec is going to support
`swift_versions` introduced in CocoaPods v1.7.0. For now, a user needs
to override `SWIFT_VERSION` appropriately in Podfile.
And add a sample code to test show(_:sender:) in ContentVC.
`FloatingPanelController.show(_:sender:)` can block 'Show' segue in a
content view controller. 'Show' segue should not be handled by
'FloatingPanelController`. So I replace this method.
Shin Yamamoto
kingcos