rb_ast_dispose does not free the rb_ast_t causing it to be leaked. This
commit changes it to use rb_ast_free instead.

For example:

    require "ripper"

    10.times do
      100_000.times do
        Ripper.sexp("")
      end

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

Before:

    27648
    32512
    37376
    42240
    47232
    52224
    57344
    62208
    67072
    71936

After:

    22784
    22784
    22784
    22784
    22912
    22912
    22912
    22912
    22912
    22912