Get Travis tests working again on PHP 7

[johnbillion]

Travis updated to PHPUnit 6 on its PHP 7 builds today. This is causing the tests on PHP 7 to fatal because core's tests are not compatible with PHPUnit 6. See #39822.

As an interim fix, PHPUnit 5.7 should be explicitly used for the PHP 7 builds.

[johnbillion]

In 40255:

Build/Test tools: Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.

This avoids fatal errors while the tests remain incompatible with PHPUnit 6, which was recently introduced on Travis.

See #39822, #40086

[johnbillion]

In 40257:

Build/Test tools: Account for PHP 5.2 when using Composer to install PHPUnit.

See #39822, #40086

[ocean90]

In 40260:

Build/Test Tools: Update .travis.yml to include latest improvements from trunk.

• Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
• On Travis CI install and use the node version which is specified in package.json.
• Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.

Merge of [40255] and [40257-40259] to the 4.7 branch.

Props netweb, johnbillion.
See #35105, #39822, #40086.

[ocean90]

In 40261:

Build/Test Tools: Update .travis.yml to include latest improvements from trunk.

• Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
• On Travis CI install and use the node version which is specified in package.json.
• Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.

Merge of [40255] and [40257-40259] to the 4.6 branch.

Props netweb, johnbillion.
See #35105, #39822, #40086.

[ocean90]

In 40262:

Build/Test Tools: Update .travis.yml to include latest improvements from trunk.

• Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
• On Travis CI install and use the node version which is specified in package.json.
• Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.

Merge of [40255] and [40257-40259] to the 4.5 branch.

Props netweb, johnbillion.
See #35105, #39822, #40086.