Add Support for WatchOS

Currently, when I try to install JSONSchema for WatchOS target via CocoaPods it says it does not support WatchOS. I see no reason why it should not support it, or I'm wrong?

`[!] The platform of the target 'TestApp_WatchUITests' (watchOS 9.0) is not compatible with 'JSONSchema (0.6.0)', which does not support 'watchOS'.`