Bump rexml to 3.4.2

https://github.com/authgear/authgear-sdk-flutter/security/dependabot/1

Author: louischan-oursky

Gemfile.lock

@@ -257,7 +257,7 @@ GEM
       trailblazer-option (>= 0.1.1, < 0.2.0)
       uber (< 0.2.0)
     retriable (3.1.2)
-    rexml (3.4.1)
+    rexml (3.4.2)
     rouge (3.28.0)
     ruby-macho (2.5.1)
     ruby2_keywords (0.0.5)