Please remove references to LLVM 3.8

[elliottslaughter]

I see references to LLVM 3.8 on the README. I plan to deprecate this version some time in the next 6-12 months. I'd like all users to switch to at least LLVM 6.0 before we do this so that we can shake out any bugs before we actually pull the plug.

[manopapad]

Blocked on StanfordLegion/legion#968