Update browser transpilation target

[jeffposnick]

We currently use @babel/preset-env to transpile down to Chrome 51:

workbox/gulp-tasks/utils/rollup-helper.js

Lines 19 to 22 in 3612461

	// This corresponds to the version of Chroumium used by
	// Samsung Internet 5.x, which is the minimum non-evergreen browser
	// we currently support.
	browsers: ['chrome >= 51'],

This target was chosen to accommodate Samsung Internet 5.x.

As per https://developer.samsung.com/internet/android/releases, there have been two stable releases of Samsung Internet 6.x based on Chromium 56 (along with a beta 7.x release based on an even newer Chromium), so I think we can safely switch that to be the new transpilation target for Workbox v4.

CC: @poshaughnessy and @AdaRoseCannon from the Samsung Internet team as a heads-up, to see if there are any concerns about shifting to Chrome 56 as the transpilation target—I'm not sure how much of a market share Samsung Internet 5.x still has.