Upgrade dependencies

Summary:
The behavior of some libraries have been modified:

• rimraf now returns a promise instead of calling a callback method
• gulp-imagemin can't be fully upgraded without switching to MJS

This prepares work for T2004, so the assets alkane recipe will be able
to build an up-to-date version of this repository to extract both
app.js and app.css, used for other infrastructure sites.

Test Plan: yarn install && yarn build

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3410