Updates the change log for Auth release 4.3.1 (#392)

* Updates the changelog for M20.

* Updates podspec version

* uses correct single quote character

Author: protocol86

Firebase/Auth/CHANGELOG.md

@@ -1,3 +1,6 @@
+# v4.3.1
+- Internal clean up.
+
 # v4.3.0
 - Provides account creation and last sign-in dates as metadata to the user
   object.

Firebase/Auth/FirebaseAuth.podspec

@@ -4,7 +4,7 @@
 
 Pod::Spec.new do |s|
   s.name             = 'FirebaseAuth'
-  s.version          = '4.3.0'
+  s.version          = '4.3.1'
   s.summary          = 'Firebase Open Source Libraries for iOS.'
 
   s.description      = <<-DESC