Silence @track_files_glob ivar warning

Fixes #446.

lib/simplecov/configuration.rb

@@ -10,6 +10,12 @@ module SimpleCov
   module Configuration # rubocop:disable ModuleLength
     attr_writer :filters, :groups, :formatter
 
+    def self.extended(base)
+      base.class_eval do
+        @track_files_glob = nil
+      end
+    end
+
     #
     # The root for the project. This defaults to the
     # current working directory.