Fixed the Travis setup for HHVM

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | -

this replaces #9997 with a proper fix: fixing the cause of the issue instead of adding workarounds for it

"php -r" does not seem to behave properly in HHVM, so the check to detect HHVM has been switch to a check on the environment variable set by Travis.