tests: use npm ci

[DanielRuf]

npm ci is much faster and is meant for CI. This PR is meant for evaluating the new ci command and testig the performance and compare the logs of Travis CI.

https://docs.npmjs.com/cli/ci
http://blog.npmjs.org/post/171556855892/introducing-npm-ci-for-faster-more-reliable

[mgol]

Thanks for the PR. However, there are a few problems with this approach:

1. We removed the lockfile on purpose in Remove package-lock.json? #3792 due to issues with it inside of npm.
2. We intentionally don't upgrade npm in CI to make sure our builds pass with npm versions included by default in respective Node.js versions as that's what usually people use.
3. This change doesn't speed up the build which is the main supposed advantage of npm ci vs npm install; compare https://travis-ci.org/jquery/jquery/builds/369211845 & https://travis-ci.org/jquery/jquery/builds/367872474

In light of the above, I don't think we'll want to switch to npm ci.

[DanielRuf]

Right. We just have to trigger a build to compare the results.

Thanks for the feedback.

[DanielRuf]

We removed the lockfile on purpose in #3792 due to issues with it inside of npm.

Which was fixen in npm 5.6 ;-) npm/npm#18135 (comment)
Also see https://yarnpkg.com/blog/2016/11/24/lockfiles-for-all/
https://medium.com/@Quigley_Ja/everything-you-wanted-to-know-about-package-lock-json-b81911aa8ab8

[mgol]

We're aware of the supposed fixes for that in npm 5.6 but #17722 is still open and there are comments indicating this is still an issue when working cross-platform.

Besides, we haven't had many problems with sub-dependencies misbehaving so it seems our contributors would suffer more from lockfile issues rather than dependencies changing under us.