Add composer packages and composer cache to Travis CI cache

[netweb]

Following up from [40269] we should add composer packages and composer cache to the Travis CI cache.

​https://docs.travis-ci.com/user/caching/

[johnbillion]

In 40538:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

Props netweb

Fixes #40539

[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

[johnbillion]

In 40548:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.6 branch.

[johnbillion]

In 40549:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.5 branch.

[johnbillion]

In 40550:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.4 branch.

[johnbillion]

In 40551:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.3 branch.

[johnbillion]

In 40552:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.2 branch.

[johnbillion]

In 40553:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.1 branch.

[johnbillion]

In 40554:

Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

See #40539

Merges [40538] to the 4.0 branch.