Crunch theme: Intelligently select between RVM and rbenv

This commit extends my **crunch** theme to make use of either RVM _or_ rbenv, depending on the list of active Oh-My-Zsh plugins.  If neither the `rvm` nor the `rbenv` plugin is active, no Ruby version is displayed.

(Note: the `rbenv` option requires the plugin from pull request #641 to be useful.)