Update SiftFlutterTtsPlugin to swift version 4.2

Author: zigadolar

ios/Classes/SwiftFlutterTtsPlugin.swift

@@ -21,7 +21,7 @@ public class SwiftFlutterTtsPlugin: NSObject, FlutterPlugin, AVSpeechSynthesizer
 
     // Allow audio playback when the Ring/Silent switch is set to silent
     do {
-      try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback)
+      try AVAudioSession.sharedInstance().setCategory(.playback)
       try AVAudioSession.sharedInstance().setActive(true)
     } catch {
       print(error)

ios/flutter_tts.podspec

@@ -16,7 +16,7 @@ A flutter text to speech plugin
   s.public_header_files = 'Classes/**/*.h'
   s.dependency 'Flutter'
   s.ios.deployment_target = '8.0'
-  s.swift_version = '4.0'
+  s.swift_version = '4.2'
   s.static_framework = true
 end