Fix the 4.2 branch test suite

[johnbillion]

The test suite for the 4.2 branch on Travis CI broke today with npm errors. The last known working (and passing) build was around a month ago.

Erroring build:
​https://travis-ci.org/WordPress/wordpress-develop/builds/295283327

Previous build (which was good):
​https://travis-ci.org/WordPress/wordpress-develop/builds/277509137

What's changed in the last month that has caused this to error? Continually fixing node dependencies for older branches is completely unmaintainable. Can we pin more dependencies to known working versions?

[johnbillion]

The same has happened on the 4.0 branch, but, curiously, not the 4.1 branch. What's different about the 4.1 branch?

[ayeshrajans]

Travis's systems had some issues yesterday (goes well with Halloween), and I had some of my own packages failing for no reason. I'll try to see if they are still failing.

[johnbillion]

@ayeshrajans Good suggestion, thanks. I just restarted the build and the same error occurred, so unfortunately that wasn't the issue.

[johnbillion]

In 44989:

Build/Test tools: Switch npm dependency caching strategy on Travis CI for the 4.2 branch.

This switches to caching npm's local cache instead of the compiled modules in node_modules. The latter is causing errors on the PHP 5.2 build.

See #42385

[johnbillion]

In 44990:

Build/Test tools: Switch npm dependency caching strategy on Travis CI for the 4.3 branch.

This switches to caching npm's local cache instead of the compiled modules in node_modules. The latter is causing errors on the PHP 5.2 build.

Merges [44989] to the 4.3 branch.

See #42385