Fix select component focus

Comment at https://github.com/Addepar/ember-widgets/blob/86a47052632ca947713ad13a7df3d867716daf16/src/select.coffee#L189:
```
    # we want to focus on search input when dropdown is opened. We need to put
    # this in a run loop to wait for the event that triggers the showDropdown
    # to finishes before trying to focus the input. Otherwise, focus when be
    # "stolen" from us.
```
Using Ember.run.next for the dropdown puts it in the same run loop as the focus call, allowing it to be "stolen"
@Dylnuge @cyril-sf