Remove ruby-install dependency and build from Ruby source

[calebhailey]

In the end, the trick which made this work was a Ruby make configure flag called load-relative which causes ruby to resolve load paths at runtime (dynamically): https://github.com/ruby/ruby/blob/d48783bb0236db505fe1205d1d9822309de53a36/configure.ac#L145-L147

The ruby-install project does provide some niceties in the form of features (and anti-features) as it describes in its readme:

## Features

* Supports installing arbitrary versions.
* Supports downloading the latest versions and checksums from [ruby-versions].
* Supports installing into `/opt/rubies/` for root and `~/.rubies/` for users
  by default.
* Supports installing into arbitrary directories.
* Supports downloading from arbitrary URLs.
* Supports downloading from mirrors.
* Supports downloading/applying patches.
* Supports specifying arbitrary `./configure` options.
* Supports downloading archives using `wget` or `curl`.
* Supports verifying downloaded archives using `md5sum`, `md5` or `openssl md5`.
* Supports installing build dependencies via the package manager:
  * [apt]
  * [dnf]
  * [yum]
  * [pacman]
  * [zypper]
  * [pkg]
  * [macports]
  * [brew]
* Has tests.

## Anti-Features

* Does not require updating every time a new Ruby version comes out.
* Does not require recipes for each individual Ruby version or configuration.
* Does not support installing trunk/HEAD.

...but we don't need all of these features. If we wanted to reduce ruby-install as an external dependency, we would likely need to refactor the Dockerfile to fetch and validate source SHAs, and recreate the required ./configure options (including --enable-load-relative).