doc: update clang version prerequisite
doc: update clang version prerequisite

See commit `4877ec0`, where it now warns on a clang++ version older than
3.4.

PR-URL: https://github.com/iojs/io.js/pull/1094
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node