Fix ruby_build_installed_verison typo.

I renamed `ruby_build_installed_verison` to `ruby_build_installed_version` in both the definition and usage.

This method is defined as public, but it looks like it's only public usage (based on a [Google search](https://www.google.com/search?q="ruby_build_installed_verison"&filter=0)) is in this recipe (and copies thereof). However, I would be happy to include a deprecated `ruby_build_installed_verison` method if you think it necessary.