doc: update clang version prerequisite

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

PR-URL: #1094
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

README.md

@@ -80,7 +80,7 @@ the binary verification command above.
 Prerequisites:
 
 * `gcc` and `g++` 4.8 or newer, or
-* `clang` and `clang++` 3.3 or newer
+* `clang` and `clang++` 3.4 or newer
 * Python 2.6 or 2.7
 * GNU Make 3.81 or newer
 * libexecinfo (FreeBSD and OpenBSD only)