This allows users who have per-project Ruby versions (e.g. with `rvm`)
to pin to a particular gem installation.
For example: `let g:ruby_host_prog = 'rvm system do neovim-ruby-host'`
The exists('g:loaded_foo') convention is rather common, and may be
relied upon in some cases. It's also very unlikely that a user or plugin
has any reason to set g:loaded_foo to zero, so the principle of least
surprise can be brushed aside here.
https://github.com/neovim/neovim/issues/6107#issuecomment-279532143
