Update a recipe of rust-racer (#2629)

recipes/rust-racer.rcp

@@ -1,10 +1,9 @@
-;; user should also set `racer-rust-src-path' to the path of the source-directory
-;; of rust in his system and also set his keybinding as described in
-;; https://github.com/phildawes/racer
+;;; currently nightly compiler is needed to install racer.
+;;; See https://github.com/racer-rust/racer for detail.
 (:name rust-racer
        :type github
-       :pkgname "phildawes/racer"
+       :pkgname "racer-rust/racer"
        :description "Rust code completion and code navigation"
-       :build '(("cargo" "build" "--release"))
+       :build '(("cargo" "+nightly" "build" "--release"))
        :prepare (setq racer-cmd (concat default-directory "/target/release/racer"))
        :post-init (add-hook 'racer-mode-hook #'eldoc-mode))