doc: upgrade Clang requirement to 3.4.2

Clang 3.4.1 has a bug that prevents compilation of V8.
As of Node.js 8.0.0 we stopped floating a workaround for this issue.

PR-URL: https://github.com/nodejs/node/pull/12388


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>