Fix chruby plugin to not complain if chruby is *not* installed

Fix for the chruby plugin that spits out error messages on shell startup if chruby is not installed (on OS X with homebrew), like this:

    <omz>/plugins/chruby/chruby.plugin.zsh:source:67: no such file or directory: /usr/local/Cellar/chruby/0.3.9/share/chruby/chruby.sh
    <omz>/plugins/chruby/chruby.plugin.zsh:source:68: no such file or directory: /usr/local/Cellar/chruby/0.3.9/share/chruby/auto.sh