Drop Node 4 support; Avoid eagerly requiring plugins and presets

[mikrostew]

This is a working version of #218, to enable parallel transpilation with the default configuration.

This adds the use of require.resolve to enable multiple independent versions of ember-cli-babel to coexist, and fixes the tests to account for the changes.

[mikrostew]

The tests are hanging because the builds are running in parallel, in worker processes that continue to run after the build completes. So mocha is hanging waiting for those processes to exit and they never do.

Since this worker pool is shared between all broccoli-babel-transpilers of the same version, shutting down the workers immediately after a build would possibly create a lot of churn.

We could shut down the worker processes after they have been idle for some time. I had thought of doing that before, to mitigate ember-cli/ember-cli#7379.

@stefanpenner @rwjblue thoughts?

[stefanpenner]

I think we should not rely on them aging out in tests

Rather we should likely explicitly kill them for now from the tests (and age out in addition seems fine) but I would hate for my tests to hang (slowing down my own productivity)

The correct solution would be to shutdown workers once the plugin or build pipeline instance that spawned them is cleanedup/destroyed. That may not be practical today (if it is we should doit) So instead, we can likely just manually kill them when the tests shutdown.

[stefanpenner]

Spoke with @rwjblue, we agreed that destroying of workers should happen in plugin.cleanup(). @mikrostew can you do that?

[stefanpenner]

Some funky windows test failures, I am:

• going to bump the timeout slightly
• consider dropping node 4 testing, since its EOL.

[stefanpenner]

released as v6.13.0 🎉

[Turbo87]

consider dropping node 4 testing, since its EOL.

☝️ that was a breaking change!

[rwjblue]

😭 we need to revert e33466e at least (and add 10 manually)...