feat(ios): Twilio Voice SDK bump 6.13.1*

https://www.twilio.com/docs/voice/sdks/ios/changelog#6131

Author: cybex-dev

ios/twilio_voice.podspec

@@ -15,7 +15,7 @@ Pod::Spec.new do |s|
   s.source           = { :path => '.' }
   s.source_files = 'Classes/**/*'
   s.dependency 'Flutter'
-  s.dependency 'TwilioVoice','~> 6.13.0'
+  s.dependency 'TwilioVoice','~> 6.13.1'
   s.platform = :ios, '11.0'
 
   # Flutter.framework does not contain a i386 slice.