Release v1.6.4 (#444)

diff --git a/CHANGELOG b/CHANGELOG
index 0882e63..ba72d15 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,10 @@
+v1.6.4
+  Version 1.6.4
+
+  Bugfix release
+
+  - Warn when google-auth credentials are used but google-auth-httplib2 isn't available. (#443)
+
 v1.6.3
   Version 1.6.3