Backport: ruby-mode: Change faces used for self, true, false and nil

Fixes: debbugs:17733

* lisp/progmodes/ruby-mode.el (ruby-font-lock-keywords): Use
`font-lock-constant-face' for nil, true and false.  Highlight
`self' as a keyword.

(cherry picked from commit e991af92)

lisp/ChangeLog

+2015-03-20  Dmitry Gutov  <dgutov@yandex.ru>
+
+	* progmodes/ruby-mode.el (ruby-font-lock-keywords): Use
+	`font-lock-constant-face' for nil, true and false.  Highlight
+	`self' as a keyword.  (Bug#17733)
+
 2015-03-20  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* progmodes/ruby-mode.el (ruby-syntax-before-regexp-re): Expect

lisp/progmodes/ruby-mode.el

@@ -2013,8 +2013,9 @@ See `font-lock-syntax-table'.")
           "rescue"
           "retry"
           "return"
-          "then"
+          "self"
           "super"
+          "then"
           "unless"
           "undef"
           "until"
@@ -2109,8 +2110,8 @@ See `font-lock-syntax-table'.")
     "\\_<\\(?:BEGIN\\|END\\)\\_>\\|^__END__$"
     ;; Variables.
     (,(concat ruby-font-lock-keyword-beg-re
-              "\\_<\\(nil\\|self\\|true\\|false\\)\\_>")
-     1 font-lock-variable-name-face)
+              "\\_<\\(nil\\|true\\|false\\)\\_>")
+     1 font-lock-constant-face)
     ;; Keywords that evaluate to certain values.
     ("\\_<__\\(?:LINE\\|ENCODING\\|FILE\\)__\\_>"
      (0 font-lock-builtin-face))