added ra.i18n.js to the precompile list because it is being included in a javascript include tag

I noticed after upgrading to the latest version I was getting a JS include error:

```
GET https://d2x5gy2xai3ykv.cloudfront.net/javascripts/rails_admin/ra.i18n.js 403 (Forbidden) 
```

On inspection I was seeing the tags look like this:

```
<link href="https://d2x5gy2xai3ykv.cloudfront.net/assets/rails_admin/rails_admin-9c0ee978c98cd0a5c14cf623c8e615f3.css" media="all" rel="stylesheet" />
<script src="https://d2x5gy2xai3ykv.cloudfront.net/assets/rails_admin/rails_admin-b906bdb9c658f8fbe44f4ea9f201d493.js"></script>
<script src="https://d2x5gy2xai3ykv.cloudfront.net/javascripts/rails_admin/ra.i18n.js"></script>
```

After adding the ra.i18n.js to the precompile list the error went away