Update README.md

clang 5.x does not exist. That's Apples distribution of llvm clang going by the name of _Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)_
+ it breaks builds for me - I have 'using clang ..' in $(boost-dir)/tools/build/v2/user-config.jam where it should be.
```apply_visitor``` interface not-compatible with boost::apply_visitor (by changing args order) wasn't smart

+ making it compatible
fix syntax
fixups