MacOS.sdk: return newest SDK if requested not found

`MacOS.sdk` and `.sdk_path` will now return the newest installed SDK
instead of nil if called on a system that doesn’t have an SDK for the
currently-installed OS. For example, Xcode 7 on OS X 10.10 does not
include the 10.10 SDK, only the 10.11 SDK; software can be built by
specifying both SDKROOT and MACOSX_DEPLOYMENT_TARGET.

Library/Homebrew/os/mac.rb

@@ -89,6 +89,9 @@ module OS
       begin
         @locator.sdk_for v
       rescue SDKLocator::NoSDKError
+        sdk = @locator.latest_sdk
+        # don't return an SDK that's older than the OS version
+        sdk unless sdk.nil? || sdk.version < version
       end
     end