switches Julia to LLVM 6.0.0 and uses BinaryBuilder for CI
- changes the support LLVM version to 6.0.0
- adds support for using a pre-built LLVM binary from
`staticfloat/LLVMBuilder` taking advantage of the
BinaryBuilder infrastructure.
- adds two Makefile flags:
- `BINARYBUILDER_TRIPLET` the BinaryBuilder triplet you are building for.
An example would be `x86_64-linux-gnu`.
- `USE_BINARYBUILDER_LLVM = 0` set this to 1 if you want to use
the pre-built binaries for LLVM.
- uses the pre-built binaries on OSX Travis instead of the bottle.
- uses the pre-built binaries on Appveyor instead of a custom cache.
