Change obsolete `%pure-parser` to `%define api.pure`.

Recent `bison` warns for `%pure-parser`. We kept it since MacOS only
provide ancient `bison`, but the warning is noisy and there's no hope
that Apple will upgrade `bison`. MacOS users must install the newer
version of `bison`, by typing `brew install bison` for example.

Note that `brew` does not overwrite the `bison` execution path
automatically, so you need to update your `.bash_profile` as instructed
by `brew`.

mrbgems/mruby-compiler/core/parse.y

@@ -1330,7 +1330,7 @@ heredoc_end(parser_state *p)
 
 %}
 
-%pure-parser
+%define api.pure
 %parse-param {parser_state *p}
 %lex-param {parser_state *p}