Add tvOS to the podspec.

Fixes https://github.com/google/protobuf/issues/3217
diff --git a/Protobuf.podspec b/Protobuf.podspec
index 649f3ae..af2e952 100644
--- a/Protobuf.podspec
+++ b/Protobuf.podspec
@@ -36,6 +36,7 @@
 
   s.ios.deployment_target = '7.0'
   s.osx.deployment_target = '10.9'
+  s.tvos.deployment_target = '9.0'
   s.watchos.deployment_target = '2.0'
   s.requires_arc = false
 end