allow use of @import 'bootstrap3-wysihtml5' in application.css.scss

Requiring the css asset from `app/assets/stylesheets/application.css.scss` with:

```sass
@import 'bootstrap-wysihtml5'
```

or

```sass
@import 'bootstrap3-wysihtml5'
```

doesn't work. You need to invoke import like this:

```sass
@import 'bootstrap-wysihtml5/bootstrap3-wysihtml5'
```

This PR allows just using:

```sass
@import 'bootstrap3-wysihtml5'
```

I fixed the `rake update` task to copy the source css to `app/assets/stylesheets`.
Tested in test app and my own project.