add basic ruby contrib layer

this contrib layer does a few things:
1. swaps `ruby-mode` for `enh-ruby-mode`
2. adds `robe-mode` to pilot `inf-ruby` and sets a few convenient keybindings
3. adds a couple of keybindings for `ruby-test-mode`
4. adds support for `rbenv`

It's all pretty basic for now, I hope I'll be able to refine it over time.
I updated the [conventions](https://github.com/syl20bnr/spacemacs/blob/develop/doc/CONVENTIONS.md) for key bindings, it gives us:
- `m t b` to test the buffer/file
- `m t f` to test the function under point