Upgrade LLVM again

It appears that LLVM has made another breaking change to our `PassWrapper.cpp` file, causing our travis builds to break. This appears to be breaking more frequently than I had anticipated, which is unfortunate.

I looked into installing and using LLVM 3.4 from their nightly repos (which is claimed to be stable), but apparently the llvm-3.4 package is missing from the repos, so it wasn't much of a success.

I'm curious what others' opinions on this are. I'm not a big fan of building LLVM all the time, but I also like having travis builds...