Introduced support for zsh on completions scripts

@sstephenson changed rbenv to support zsh (zsh does not support the complete command). It was added around commit [f635c8715cd990fa6f066117c67240ac38ef9b46](https://github.com/sstephenson/rbenv/commit/f635c8715cd990fa6f066117c67240ac38ef9b46#completions/rbenv.zsh).

This PR adds the same support to the cookbook, otherwise you'll get a error message when loggin onto the machine.

`/opt/rbenv/completions/rbenv.bash:16: command not found: complete`