Fixed the Travis setup for HHVM
minor #9999 Fixed the Travis setup for HHVM (stof)

This PR was squashed before being merged into the 2.3 branch (closes #9999).

Discussion
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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.

Commits
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67d4959 Fixed the Travis setup for HHVM