fix(start()): locate parent before every 'start()'

When the ng-view is attached to the body
And the route is changed
Then the body may be re-written out from under the loading bar
And the $parent pointer will be left in a bad state

By finding the $parent every time the loading bar is started,
it ensures that the loading bar's <div> elements aren't attached
to a stale parent.

I discovered this bug on a project I'm working on.  I tested this
fix, and it seemed to do the trick.  The loading bar still vanishes
part-way through a route change, but it begins working again
the next time start() is called.  (Prior to this fix, it did not.)

More details:  We're using ui-router and attaching our app to the
body element.  <body ui-view>...</body>