Remove HHVM from the test matrix on Travis

[netweb]

Via ​#WP40548

Maintaining the HHVM test infrastructure on Travis takes a disproportionately high amount of effort. Particularly because the HHVM build:

• Is in the allowed failures list
• Has never passed
• Often errors

Given the exceptionally low usage numbers of HHVM, support in the Travis test suite should be removed.

The PHP world owes a lot to HHVM for helping it push it forward. Without HHVM, maybe we wouldn't have seen such incredible performance gains in PHP 7. Thank you, HHVM.

[netweb]

In 6405:

Build Tools: Remove HHVM from the test infrastructure on Travis.

Fixes #3104.