Ensure inline lists uses the right Bootstrap 3 css class

According to the bootstrap migration readme http://getbootstrap.com/migration/ - the .inline class is deprecated on inline lists in favour of .list-inline.

The upshot of this for rails_admin is that the action icons on the index and dashboard page currently look 'squished' together.

This commit adds the .list-inline class  but also keeps the .inline class around for backwards compatibility. 

I notice that this is a common pattern in rails_admin for moving to Bootstrap 3, as there are several deprecated css classes dotted around the place. At some point these should be removed (moving to Bootstrap 3 *is* a breaking change)