Replace uglify with terser to support ES6 output

[nanek]

Type of change

• Build related changes

Description of change

Changing the babel presets-env browser target to just Chrome in the master branch will fail with an error.

[19:03:38] 'makeWebpackPkg' errored after 8.06 s
[19:03:38] GulpUglifyError: unable to minify JavaScript
Caused by: SyntaxError: Unexpected token: keyword «const»
File: /Users/kenan/scm/sd/Prebid.js/criteoBidAdapter.js
Line: 38
[19:03:38] 'build-bundle-prod' errored after 8.07 s
[19:03:38] 'build' errored after 8.13 s

uglify-js does not support ES6+. By switching to terser, builds of prebid can target es6 if desired.

Other information

https://github.com/terser/terser#why-choose-terser

[nanek]

Not related to using terser, however this fixes other build errors thrown by webpackStream if you target just Chrome. @babel/plugin-proposal-object-rest-spread is part of babel/preset-env.

[ChrisHuie]

@nanek looks like our tests didn't run on this pr but I can test it out in an updated dependency pr #6828 that is about to get merged. We are looking at upgrading webpack but it is taking a bit of effort to convert to a chunking plugin that is now built into webpack

[ChrisHuie]

closing as these changes have been added to #6828