[ruby/stringio] Revert "Bump version to 3.0.4"

This reverts commit https://github.com/ruby/stringio/commit/aeb7e1a0bde6.

https://github.com/ruby/stringio/commit/003dd0d003

ext/stringio/stringio.c

@@ -12,7 +12,7 @@
 
 **********************************************************************/
 
-#define STRINGIO_VERSION "3.0.4"
+#define STRINGIO_VERSION "3.0.3"
 
 #include "ruby.h"
 #include "ruby/io.h"