fix(doc-gen): Run Gulp on Windows too

Using node_module/.bin/gulp will enable to gulp command to run
both on Windows and Linux. In its current form, the default action of
executing a Javascript file on Windows does not use node.
Requires quotes around the command to correctly resolve path on Windows

Closes #6346

lib/grunt/plugins.js

@@ -35,7 +35,7 @@ module.exports = function(grunt) {
 
 
   grunt.registerTask('docs', 'create angular docs', function(){
-    var gruntProc = shelljs.exec('node_modules/gulp/bin/gulp.js --gulpfile docs/gulpfile.js');
+    var gruntProc = shelljs.exec('"node_modules/.bin/gulp" --gulpfile docs/gulpfile.js');
     if (gruntProc.code !== 0) {
       throw new Error('doc generation failed');
     }