Require LLVM 3.6 for Rust 0.13.0-dev.

ExecutionEngineWrapper.cpp needs LLVM to be 3.6 because of an
ABI-incompatible change of that.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #40 from omasanori/llvm36

Require LLVM 3.6 for Rust 0.13.0-dev.