Build: Drop support for building on Node.js 0.12

Build: Drop support for building on Node.js 0.12

Node.js 0.12 is no longer supported by upstream. So far we had to keep
a workaround that would skip running ESLint there (or even load its task)
as it no longer supports Node.js <4; the same applied to Node smoke tests
as jsdom has also dropped support for those old Node versions. Those
workarounds have been removed now.

Ref 030191ae32dfcb7ecb2efb61d17a4964a3633e44
Closes gh-3478