Upgrade test suite to use selenium-webdriver v4

[jeffposnick]

This PR updates our dependency on selenium-assistant to v6.0.0, which supports a version of the underlying selenium-webdriver project that will work with Safari.

It also changes our Travis CI config to use an OS X container instead of Linux, which will result in our Safari tests being run as part of our CI. (This unfortunately takes a long-ish time to complete.)

To work around some issues related to chromedriver (and geckodriver) versioning, it removes them from package.json and instead installs them fresh before every gulp test-integration.

(There's a somewhat mysterious increase in size in the packages/workbox-window/build/workbox-window.dev.umd.js bundle reported by the PR-Bot Size Plugin, and I'm not sure what's up with that? It might be because the various Babel-related dependencies that were locked to earlier versions in our old package-lock.json have been updated?)

[coveralls]

Coverage remained the same at 78.731% when pulling 8519318 on selenium-revamp into fec46ef on master.

[philipwalton]

What's the story here?

[jeffposnick]

As per https://docs.travis-ci.com/user/caching/#npm-cache, it should happen automatically now.

[workbox-pr-bot]

PR-Bot Size Plugin

Changed File Sizes

File	Before	After	Change	GZipped
packages/workbox-window/build/workbox-window.dev.umd.js	31.88 KB	40.32 KB	+26%	8.90 KB	☠️
packages/workbox-window/build/workbox-window.prod.umd.js	4.46 KB	4.51 KB	+1%	1.85 KB

New Files

No new files have been added.

All File Sizes

View Table

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.86 KB	3.86 KB	0%	1.59 KB
packages/workbox-broadcast-update/build/workbox-broadcast-update.prod.js	1.92 KB	1.92 KB	0%	959 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	3.30 KB	3.30 KB	0%	1.31 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	594 B	594 B	0%	354 B
packages/workbox-cli/build/app.js	4.16 KB	4.16 KB	0%	1.64 KB
packages/workbox-cli/build/bin.js	940 B	940 B	0%	502 B
packages/workbox-core/build/workbox-core.prod.js	5.94 KB	5.94 KB	0%	2.46 KB
packages/workbox-expiration/build/workbox-expiration.prod.js	2.94 KB	2.94 KB	0%	1.27 KB
packages/workbox-google-analytics/build/workbox-offline-ga.prod.js	1.95 KB	1.95 KB	0%	913 B
packages/workbox-navigation-preload/build/workbox-navigation-preload.prod.js	660 B	660 B	0%	324 B
packages/workbox-precaching/build/workbox-precaching.prod.js	4.89 KB	4.89 KB	0%	1.90 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.57 KB	1.57 KB	0%	797 B
packages/workbox-routing/build/workbox-routing.prod.js	3.09 KB	3.09 KB	0%	1.35 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	4.10 KB	4.10 KB	0%	1.13 KB
packages/workbox-streams/build/workbox-streams.prod.js	1.46 KB	1.46 KB	0%	720 B
packages/workbox-sw/build/workbox-sw.js	1.34 KB	1.34 KB	0%	747 B
packages/workbox-webpack-plugin/build/generate-sw.js	3.71 KB	3.71 KB	0%	1.40 KB
packages/workbox-webpack-plugin/build/index.js	349 B	349 B	0%	255 B
packages/workbox-webpack-plugin/build/inject-manifest.js	4.68 KB	4.68 KB	0%	1.59 KB
packages/workbox-window/build/workbox-window.dev.umd.js	31.88 KB	40.32 KB	+26%	8.90 KB	☠️
packages/workbox-window/build/workbox-window.prod.umd.js	4.46 KB	4.51 KB	+1%	1.85 KB

Workbox Aggregate Size Plugin

3.5KB gzip'ed (23% of limit)
7.79KB uncompressed

[jeffposnick]

(Here's the before-and-after of workbox-window.dev.umd.js: https://gist.github.com/jeffposnick/1168ba18561b94b82233d64f5d5521ac/revisions?diff=split. They're too big for the built-in gist diff viewer, so you need to download them.)

[jeffposnick]

Based on eyeballing the diff locally, it looks like the bulk of the changes in the workbox-window dev UMD bundle is due to the inclusion of some JSDocs that were not included in the previous bundle, related to events; e.g.:

   /**
     * The `externalactivated` event is dispatched if the state of an
     * [external service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-external-sw}
     * changes to `activated`.
     *
     * @event module:workbox-window.Workbox#externalactivated
     * @type {WorkboxEvent}
     * @property {ServiceWorker} sw The service worker instance.
     * @property {Event} originalEvent The original [`statechange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/onstatechange}
     *     event.
     * @property {string} type `externalactivated`.
     * @property {Workbox} target The `Workbox` instance.
     */