LLVM 5.0 is now the master branch

[ianamason]

• [ ] We need to build 5.0 and fudge a travis install
• [ ] Fix travis for all the older LLVM version branches that are failing.
• [ ] Argv specialization
• [ ] razor (aka slash) not picking the right clang++
• [ ] hello vs argv small perturbation in the C causing crashes

[caballa]

@ianamason : What is the status of this? travis seems to work. We don't need to fix travis for older LLVM versions. What about the rest?