Chruby support

[Immortalin]

Needs documentation.

[StreakyCobra]

#2321 Is about chruby too, so if one get merged, the other should be closed.

[StreakyCobra]

It would be nice to document the chruby-ruby-version variable in the README. Not all users will read the source code :-)

I also have a question, you didn't added chruby to the list of packages because it's provided by another package? Or did I missed something?

[Immortalin]

@StreakyCobra aren't you suppose to set the ruby-version-manager variable at runtime I.e. in your .spacemacs.d? I haven't had time to add in the documentation for usage :(

[StreakyCobra]

I'm not a ruby guy, so I don't know how all of this work 😃 I was just asking, I'll let other people have a look at it :-)

[syl20bnr]

Thank you.
I made some changes to make chruby behaves like rbenv and rvm, see documentation here: https://github.com/syl20bnr/spacemacs/tree/develop/layers/%2Blang/ruby#ruby-version-management
Cherry-picked into develop branch, you can safely delete your branch.