llvm34: Fix head url

The LLVM trunk has already been bumped to version 3.5,
and the LLVM 3.4 is currently in a pending release phase
(which means that there are no tarballs available yet).

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
llvm35: New formula (trunk, head only)

Closes #289.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>