lang/clojure-mode.el: Update to 5.7.0

While here, update LICENSE and improve pkg-descr, WWW wasn't
pointing to the project address.

Changes: https://github.com/clojure-emacs/clojure-mode/blob/5.7.0/CHANGELOG.md#570-2018-04-29

PR: 227884
Submitted by: harrigan-s@outlook.com (maintainer)