@ symbol shouldn't be a word character

Unless @ symbol is treated as a non-word character, symbols that are being dereferenced aren't returned in a usable fashion by `(symbol-at-point)`

See discussion on cider pull request https://github.com/clojure-emacs/cider/pull/887 and cider issue https://github.com/clojure-emacs/cider/issues/886 (which is really a clojure-mode issue, apparently)