Travis CI cached NPM module maintenance

[netweb]

Previously in #36291 via r37058 caching NPM modules in Travis CI was added.

Caching modules in Travis CI the cache needs to be maintained so it doesn't grow exponentially with each new, updated, or removed NPM module.

Including npm prune facilitates this by only caching the current modules installed by npm install

[ocean90]

In 37340:

Travis CI: Include npm prune in the before_script command.

Caching of node modules was added in [37058]. npm prune removes extraneous packages so the cache contains only current modules.

Props netweb.
Fixes #36490.

[ocean90]

In 40280:

Build/Test Tools: Allow Travis CI to cache the node_modules directory.

Merge of [37058] and [36490] to the 4.4 branch.

See #36291, #36490.

[ocean90]

In 40281:

Build/Test Tools: Allow Travis CI to cache the node_modules directory.

Merge of [37058] and [36490] to the 4.3 branch.

See #36291, #36490.

[ocean90]

In 40282:

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.
• Get Travis builds working on HHVM again.
• In addition, brings the Slack notification settings up to date and allows Travis CI to cache the node_modules directory.

Merge of [40255], [40257-40259], [40269], and [40271] to the 4.2 branch.

See #35105, #40100, #30755, #36291, #36490.

[ocean90]

In 40286:

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

• On Travis CI install and use node version 0.10.48 to ensure correct node-sass builds.
• Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
• Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.
• Get Travis builds working on HHVM again.
• In addition, brings the Slack notification settings up to date and allows Travis CI to cache the node_modules directory.

Merge of [40255], [40257],[40259], [40269], and [40271] to the 4.1 branch.

See #35105, #40100, #30755, #36291, #36490.

[ocean90]

In 40288:

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

• On Travis CI install and use node version 0.10.48 to ensure correct node-sass builds.
• Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
• Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.
• Get Travis builds working on HHVM again.
• In addition, brings the Slack notification settings up to date and allows Travis CI to cache the node_modules directory.

Merge of [40255], [40257],[40259], [40269], and [40271] to the 4.0 branch.

See #35105, #40100, #30755, #36291, #36490.