Ruby

Fix memory leak in Ripper.sexp

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