upgrade uglifyjs due to nodejs security advisories

[grawk]

https://nodesecurity.io/advisories/39
https://nodesecurity.io/advisories/48

Update to version 2.6.0 or later

[gabrielcsapo]

could be moved to use https://github.com/ternjs/acorn

[grawk]

I spent some time trying to understand how to hook up the new version of uglify. I'll admit it's beyond my ability (or at least within the time I spent) to understand what's going on in this module 😆

If someone else wants to take a crack at adapting acorn or the newer version of uglify, that'd be awesome.

But in general, since this module appears to be drifting, it's probably best to move away from it entirely unless someone decides to adopt it.