[WIP] Fixed .travis.yml for hhvm

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | -

A directory seems missing. See https://travis-ci.org/symfony/symfony/jobs/16726592