Fix warnings for contrib/slime-clipboard.el.

* contrib/slime-clipboard.el: Require `cl-lib`/`cl` for `lexical-let` and `cl-`
prefix.
(slime-clipboard-insert-entries, slime-clipboard-insert-ref)
(slime-clipboard-ref-modified): Use `cl-` prefix.
Merge pull request #153 from Ferada/fix-clipboard

Fix warnings for contrib/slime-clipboard.el.