problem installing LLVM 3.9 on ubuntu with newer LLVM already installed #1523
Description
One of our reviewers ran into an issue where he had LLVM 5 installed in ubuntu 15.4 and wasn't able to install ponyc because our instructions in book/getting-started/linux-setup.md tell the user to use LLVM 3.9, which he wasn't able to install correctly.
We should investigate what this issue is here and figure out if there's a way we can give the user instructions for how to handle this situation.