Avoid duplicate loading of JavaScript in Rails4

Layout template that is generated by the following command

```
rails g bootstrap:layout [LAYOUT_NAME] [*fixed or fluid]
```

includes `javascript_include_tag` in the body tag.

In rails4, turbolinks replace body contents instead of reload page,
so JavaScript loaded multiple times.

If target page has submit button with `data-confirm="Are You Sure?"`,
confirm dialog shown multiple times.