[coffeescript mode] Stop highlighting .keyword (.on/.off/.in) as error

Closes #1779

mode/coffeescript/coffeescript.js

@@ -259,7 +259,7 @@ CodeMirror.defineMode('coffeescript', function(conf) {
         if (current === '.') {
             style = state.tokenize(stream, state);
             current = stream.current();
-            if (style === 'variable') {
+            if (/^\.[\w$]+$/.test(current)) {
                 return 'variable';
             } else {
                 return ERRORCLASS;