Fix memory leak in ruby_parser

For example:

    10.times do
      100_000.times do
        eval("")
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    19872
    26480
    32848
    39504
    45904
    52672
    59200
    65760
    72128
    78496

After:

    17328
    20752
    23664
    28400
    30656
    34224
    37424
    40784
    43328
    46656

ruby_parser.c

@@ -518,6 +518,7 @@ parser_free(void *ptr)
 {
     struct ruby_parser *parser = (struct ruby_parser*)ptr;
     rb_ruby_parser_free(parser->parser_params);
+    xfree(parser);
 }
 
 static size_t