Changelog and podspec for GoogleUtilities 6.3.1 (#3956)

GoogleUtilities.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'GoogleUtilities'
-  s.version          = '6.3.0'
+  s.version          = '6.3.1'
   s.summary          = 'Google Utilities for iOS (plus community support for macOS and tvOS)'
 
   s.description      = <<-DESC

GoogleUtilities/CHANGELOG.md

@@ -1,3 +1,7 @@
+# 6.3.1
+- Fix GULMutableDictionary keyed subscript methods. (#3882)
+- Update Networking to receive data for POST requests. (#3940)
+
 # 6.3.0
 - GULSecureCoding introduced. (#3707)
 - Mark unused variables. (#3854)