When rbenv is used for managing Ruby installations, just checking for an
executable called "neovim-ruby-host" is not enough. It has to be run as well.
If it does not return 0, then neovim-ruby-host is merely a shim for another Ruby
installation.
$ rbenv versions
2.5.0
* 2.5.1 (set by /Users/mhi/.rbenv/version)
$ rbenv whence neovim-ruby-host
2.5.0
$ which neovim-ruby-host
/Users/mhi/.rbenv/shims/neovim-ruby-host
$ neovim-ruby-host
rbenv: neovim-ruby-host: command not found
The `neovim-ruby-host' command exists in these Ruby versions:
2.5.0
$ echo $?
127
Additionally, the detection logic was moved from provider#ruby#Detect() to
s:detect(), because the former is run in the sandbox which forbids calling
system().