graal-core and truffle have been merged into new repo

[Doug Simon]

To simplify working with Graal, we have consolidated https://github.com/graalvm/graal-core and https://github.com/graalvm/truffle into https://github.com/graalvm/graal. We will be updating the documentation in this new repo in the next few days. In the mean time, new issues and PRs should only be made on https://github.com/graalvm/graal.

-Doug