Add parallel compilation

This passes the `--jobs` flag to `make` which allows it to compile files in parallel. On large machines, this can cut down the compilation time many, many times. It receives the number of cores from running `nproc`, which obeys to the role being compatible with Debian and Ubuntu. Parallel compilation of Ruby is entirely safe (e.g. `ruby-build` does it by default).

Also adds `--disable-install-doc` which disables generating `ri` and `rdoc` documentation which is not particularly useful in deployment settings.

I've tested this with a local Vagrant and it appears to work!