caddr and cdddr are not defined by default

In enh-ruby-mode.el, [caddr](https://github.com/zenspider/enhanced-ruby-mode/blob/2736dedbe315c21b5e75e5800a0c36be90496d65/enh-ruby-mode.el#L1013) and [cdddr](https://github.com/zenspider/enhanced-ruby-mode/blob/2736dedbe315c21b5e75e5800a0c36be90496d65/enh-ruby-mode.el#L1025) are used, but they are not defined by default. Because in most cases other libraries require `cl`, which defines them, this problem doesn't come out.

This pull request adds `(require 'cl)` to enh-ruby-mode.el.