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

This reverts commit 682a31f9.
Unfortunately, I couldn't run newer `bison` on TravisCI. Maybe next
time.

mrbgems/mruby-compiler/core/parse.y

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