Update ios_check_beta_deps to work with immutable strings

Author: mokagio

lib/fastlane/plugin/wpmreleasetoolkit/actions/ios/ios_check_beta_deps.rb

@@ -33,7 +33,7 @@ def self.run(params)
           end
         end
 
-        message = ''
+        message = String.new
         if non_stable_pods.empty?
           message << ALL_PODS_STABLE_MESSAGE
         else