update readme minimal clang version

README.md

@@ -26,7 +26,7 @@ Project is pure C++ builded by makefile. Some tests are handled by
 shell scripts.
  * `make >= 3.9`
- * `g++ >= 4.8` or `clang++ >= 3.5`
+ * `g++ >= 4.8` or `clang++ >= 4.0`
  * `libcppunit-dev`
  * `libtclap-dev`
  * `libxml2-dev`