Update min iOS version in Objc sample podfile (#271)

Samples/ObjC/SignInSample/Podfile

@@ -1,4 +1,4 @@
-platform :ios, '9.0'
+platform :ios, '10.0'
 
 target 'SampleForPod' do
   pod 'GoogleSignIn', :path => '../../../', :testspecs => ['unit']