Running composer on Travis with PHP set to "hhvm" with an otherwise stock config results in this error:

$ composer self-update
Fatal error: Uncaught Error: Keyword __HALT_COMPILER must be written in lowercase in /home/travis/.phpenv/versions/hhvm-stable/bin/composer:27

HHVM version is 4.3.0; not sure about Composer version as composer --version dies with the same error, but in non-HHVM builds it is 1.8.5, so I'd assume it's the same.