* .travis.yml (env): Remove emacs23 check target

* swank-loader.lisp (sly-version-string): Greps for version in
sly.el.

* sly.el: Rename existing "Slime" references to SLY. Just require
`cl-lib' normally. Error out on Emacs versions <= 24.3.

* contrib/sly-asdf.el (fboundp): Simplify fbound tricks for
earlier Emacs version.

* contrib/sly-fuzzy.el (sly-fuzzy-complete-symbol): Don't need to
use `comint-filename-completion'.

* doc/slime.texi (Platforms): Remove Emacs 23 support.