Add dep for task gulp scripts:watch

With Webpack (ES6) `gulp watch` use 'scripts:watch'  and 'scripts'
These two tasks inject module preloader and loader to `.tmp/serve/app/index.js`

To fix #457 gulp should wait the end of 'scripts' before launch 'scripts:watch'

Author: Mehdy Dara

app/templates/gulp/_scripts.js

@@ -88,7 +88,7 @@ module.exports = function(options) {
     return webpack(false);
   });
 
-  gulp.task('scripts:watch', function (callback) {
+  gulp.task('scripts:watch', ['scripts'], function (callback) {
     return webpack(true, callback);
   });
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