Fix file.exists in new Ruby version

Author: sergiomb2

lib/svn2git/migration.rb

@@ -70,7 +70,7 @@ def parse(args)
       options[:rebasebranch] = false
       options[:forceenustogit] = false
 
-      if File.exists?(File.expand_path(DEFAULT_AUTHORS_FILE))
+      if File.exist?(File.expand_path(DEFAULT_AUTHORS_FILE))
         options[:authors] = DEFAULT_AUTHORS_FILE
       end