Set ios_deployment_target to iOS 11.0 (#574)

build/config/ios/ios_sdk.gni

diff --git a/build/config/ios/ios_sdk.gni b/build/config/ios/ios_sdk.gni
index 1ff304d..fa9a903 100644
--- a/build/config/ios/ios_sdk.gni
+++ b/build/config/ios/ios_sdk.gni
@@ -16,7 +16,7 @@
   use_ios_simulator = true
 
   # Version of iOS that we're targeting.
-  ios_deployment_target = "9.0"
+  ios_deployment_target = "11.0"
 
   # The path to the iOS device SDK.
   ios_device_sdk_path = ""