Support browserify `>= 16`

[bendrucker]

Tests are failing against browserify 16 and 17:

• test/bundle-ember-bower.js
• test/bundle-pack-bower.js

In order to officially support these versions we would need tests to pass. Peer dependencies are advisory (warnings) in modern npm versions rather than requirements (errors) so users who don't care about the broken functionality can still use browserify-shim with newer versions of browserify.