Remove references to Node 4 compat

[thymikee]

Do you want to request a feature or report a bug?
Chore after Jest 22 release

There are multiple references, notes and tips regarding getting some features to work with Node4 and npm 2. Since we drop these in v22, it's safe to remove them after the release.

We should also make it clear and explicit in README, website and package.json's engines entry that Jest doesn't work with Node 4 (and npm 2).

[genintho]

Is Node 6 the new minima? I am asking to update the Troubleshooting section http://facebook.github.io/jest/docs/en/troubleshooting.html#compatibility-issues

[thymikee]

Yes and no. We decided to drop Node 4 support by default, because we upgraded to the new JSDOM. However, we still compile to Node 4-compliant code (but don't test it on CI though...), so it's possible to use it, but you need a custom test environment. See this issue for more details: #5211

[satya164]

I'm working on this one.

[github-actions]

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