update to latest commit of im-rs

[dwightguth]

bodil/im-rs#55 is fixed so we should update to that commit and uncomment the regression test that I wrote, so that we can run the iele test that failed.

[dwightguth]

Unfortunately because the latest release of im-rc uses "edition 2018" of rust, which is first available in rust 1.31, we need to update to the latest version of rustc to accomplish this. Since we want to continue to use llvm 6, this means building rust from source, which I am working on now. I have rust built, but not cargo. Still waiting to hear back from a message sent on the rust discord about how to bootstrap cargo.