Drop hhvm 3.18 CI

It has segfaults on each CI run
https://travis-ci.org/github/phpmyadmin/sql-parser/jobs/742505329#L528

Signed-off-by: William Desportes <williamdes@wdes.fr>

Author: williamdes

.travis.yml

@@ -12,7 +12,6 @@ php:
   - 7.3
   - 7.4
   - hhvm-3.12
-  - hhvm-3.18
   - hhvm-3.24
   - nightly