llvm34: Fix head url
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 <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

Ashton-W/homebrew-versions,coeligena/homebrew-verscustomized,DomT4/homebrew-versions,jimhe/homebrew-versions,heuermh/homebrew-versions,UniqMartin/homebrew-versions