Find Ruby's headers by using rbconfig
`--impl-debug true` doesn't work with bindgen 0.48.1
Less hard-coding on scripts/bindgen.sh
Still ruby has to be cloned as ~/clones/ruby, but rbspy doesn't have
to cloned as ~/work/rbspy.
Fix "warning: unused `std::result::Result` that must be used"
Since Rust 1.28.0, unit tests can return Result.
Upgrade crates to fix #198
The warning is coming from clap's `arg_enum!` macro, which has been