Fix FrozenError 🐛 on authentication errors

This fixes the following error:

     FrozenError:
       can't modify frozen String

Now that we've added frozen string literal comments we need to be aware
of string mutations. It appears this is the only mutation we make.

Author: cupakromer

lib/kracken/json_api/public_exceptions.rb

@@ -15,7 +15,8 @@ def log_error(env, wrapper)
         trace = wrapper.framework_trace if trace.empty?
 
         ActiveSupport::Deprecation.silence do
-          message = "\n#{exception.class} (#{exception.message}):\n"
+          # After Ruby 2.2.3 support change `String.new` to `+` literal
+          message = String.new("\n#{exception.class} (#{exception.message}):\n")
           message << exception.annoted_source_code.to_s if exception.respond_to?(:annoted_source_code)
           message << "  " << trace.join("\n  ")
           logger.fatal("#{message}\n\n")