oniguruma: bump to version 6.9.4

This version adds a new RegSet API, and fixes the following:
 - CVE-2019-19012
 - CVE-2019-19203
 - CVE-2019-19204
 - CVE-2019-19246
 - some problems (found by libFuzzer test)

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>