Update platform iOS to 10.0 in podspec

According to React core and React podspec right now is `10.0` should it better to set same version, it will resolve warning about target version.

(React-Core.podspec)[https://github.com/facebook/react-native/blob/master/React-Core.podspec]
(React.podspec)[https://github.com/facebook/react-native/blob/master/React.podspec]

Author: suraneti

react-native-app-auth.podspec

@@ -9,7 +9,7 @@ Pod::Spec.new do |s|
   s.license      = package['license']
   s.authors      = package['author']
   s.homepage     = package['homepage']
-  s.platform     = :ios, '9.0'
+  s.platform     = :ios, '10.0'
   s.source       = { :git => 'https://github.com/FormidableLabs/react-native-app-auth.git' }
   s.source_files  = 'ios/**/*.{h,m}'
   s.requires_arc = true