Set ios_deployment_version to iOS 8 (flutter#71)

To be followed by a patch to tools/gn in the engine to eliminate
overriding this value.

Author: cbracken

build/config/ios/ios_sdk.gni

@@ -14,7 +14,7 @@ declare_args() {
   use_ios_simulator = true
 
   # Version of iOS that we're targeting.
-  ios_deployment_target = "6.0"
+  ios_deployment_target = "8.0"
 
   # The iOS Code signing identity to use
   ios_code_signing_identity = ""