swift234

README: Add Ubuntu 14.04 clang-3.6 dependency note

We have some C++14 build dependencies, so we need at least clang-3.6.
clang-3.6 on Ubuntu 14.04 LTS doesn't lay down /usr/bin/clang++ ->
/usr/bin/clang++-3.6 but the Swift frontend requires the clang driver
to be at that path.