build(gulp-uglify): use new syntax for preserving some comments

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

The gulp-uglify update broke the build-js task due to a breaking change to the gulp-uglify API.

Issue Number:
N/A

What is the new behavior?

Fixed build broken by gulp-uglify update.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Example of a broken build: https://travis-ci.org/angular/material/jobs/401935217

[Splaktar]

The only Travis CI failure here appears to be due to a bad or in-progress AngularJS SNAPSHOT. It can be ignored.