Remove traling slashes from namespace if present

When you provide an empty string for the admin namespace, but provided a namespace within your adapter then you get a resulting namespace with a trailing slash which leads to generate a url with two slashes.

Example:
application adapter namespace:
```
namespace: 'api/v1',
host: 'http://localhost:1337'
```
admin namespace:
`namespace: ''`
leads to `http://localhost:1337/api/v1//model`

The replace makes sure that the last slash in the namespace gets removed and this then leads to the correct url `http://localhost:1337/api/v1/model`
Sure, you can see here: https://github.com/dockyard/ember-admin/blob/master/addon/components/admin-input.js#L3 and here: https://github.com/dockyard/ember-admin/blob/master/addon/components/admin-input.js#L13