Change la commande utilisée pour cconstuire reveal.js.

Gruntfile.js

 const loadGruntTasks = require("load-grunt-tasks");
+const { sep } = require("path");
 
 module.exports = grunt => {
   const BUILD_DIR = "build/";
@@ -46,7 +47,7 @@ module.exports = grunt => {
         }
       },
       buildRevealJs: {
-        command: "node_modules/.bin/grunt css jshint",
+        command: `sh node_modules${sep}.bin${sep}grunt css jshint`,
         options: {
           cwd: BUILD_DIR
         }