Add watchOS 3.0 availability for urlSession(:task:didFinishCollecting:)

[SimonRice]

I've updated to the latest version of Apollo iOS today on a project I have in the works 🎉.

However, when set up via SPM on a watchOS-only project, the main Apollo library wouldn't build because of this error:

/path/to/apollo-ios/Sources/Apollo/URLSessionClient.swift:115:53: error: 'URLSessionTaskMetrics' is only available in watchOS 3.0 or newer
                       didFinishCollecting metrics: URLSessionTaskMetrics) {
                                                    ^
/path/to/apollo-ios/Sources/Apollo/URLSessionClient.swift:113:13: note: add @available attribute to enclosing instance method
  open func urlSession(_ session: URLSession,
            ^
/path/to/apollo-ios/Sources/Apollo/URLSessionClient.swift:11:12: note: add @available attribute to enclosing class
open class URLSessionClient: NSObject, URLSessionDelegate, URLSessionTaskDelegate, URLSessionDataDelegate {

In this PR, I've opted for adding watchOS 3.0 to the existing @available attribute on the instance method.

[designatednerd]

Gah, I thought I got all these but apparently not. Good catch!