Refactor rubies

Remove duplication of interpreters tasks and use defined structure to build them.

This allow adding extra new interpreters without issues (about to add Ruby 2.2.0-preview2)

/cc @jonforums @shirosaki 
Actually, this was refactored in newer revisions of the rubyXX script, so it end being:

```ruby
        # remove path reference to sandbox (after install!!!)
        rbconfig = Dir.glob("#{package.install_target}/lib/**/rbconfig.rb").first
        contents = File.read(rbconfig).gsub(/#{Regexp.escape(full_install_target)}/) { |match| "" }
```

Issue was present in Ruby 1.8.6, as commented in 4362be94db00741a540e21a3d0c32917219f9bec

Since we no longer support 1.8.6, and I don't expect any new 1.8.7 go out, I think it is safe to ditch this customization :grin: