travis: Add testing in HHVM

Switch executable test to use "php -r 'echo PHP_VERSION_ID;'" instead of
"php -v" because under HHVM, the -v option returns the HHVM version:
> HipHop VM 3.3.0-dev (rel)

Not PHP equivalent. As a bonus, it simplifies the logic by no longer
parsing a version number but getting the intended value directly.