Remove HHVM from the test matrix on Travis

[johnbillion]

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.

[johnbillion]

In 40546:

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

Fixes #40548

[johnbillion]

In 40547:

Build/Test Tools: Backport various recent changes to the 4.7 branch.

• Add support for PHPUnit 6+.
• Add Composer files to the cache on Travis.
• Remove HHVM from the test infrastructure on Travis.

Merges [40536], [40538], [40539], and [40546] to the 4.7 branch.

See #40539
Fixes #39822, #40548

[netweb]

Reopening, branches back to 4.0 still contain remnants of HHVM in .travis.yml, we should remove all these.

[netweb]

Patch 40548.diff​ removes .travis.yml HHVM references in the 4.0-4.6 branches via [40536] / [40546] changesets

[obenland]

@johnbillion Could you take another look here?

[johnbillion]

In 40818:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.6 branch.

Props netweb

See #40548

[obenland]

@johnbillion Can this be closed?

[johnbillion]

A few more branch to commits to go then we'll be done.

[johnbillion]

In 40828:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.5 branch.

Props netweb

See #40548

[johnbillion]

In 40829:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.4 branch.

See #40548

[johnbillion]

In 40830:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.3 branch.

See #40548

[johnbillion]

In 40831:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.2 branch.

See #40548

[johnbillion]

In 40832:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.1 branch.

See #40548

[johnbillion]

In 40833:

Build/Test Tools: Remove mentions of HHVM from the test infrastructure on Travis for the 4.0 branch.

See #40548