Add now required flutter post install call to Podfile (#138)

Also update minimum iOS version in Podfile.

Author: parlough

newsfeed_ios/Podfile

@@ -1,5 +1,5 @@
 # Uncomment the next line to define a global platform for your project
-platform :ios, '9.0'
+platform :ios, '11.0'
 
 target 'newsfeed_app' do
   # Comment the next line if you don't want to use dynamic frameworks
@@ -22,3 +22,7 @@ target 'newsfeed_app' do
   end
 
 end
+
+post_install do |installer|
+  flutter_post_install(installer) if defined?(flutter_post_install)
+end