Fixed multiple observers alive

android/architecture-components-samples#47
As pointed out in that blogpost, everything's fine if a fragment is
destroyed (after a rotation for example), however if it's
detached/attached then the observer isn't removed and a new one is
created every time - unless we use `viewLifecycleOwner` instead of
`this` as the observation's owner.

app/src/main/java/me/gobetti/codechallenge/modules/list/ListFragment.kt

@@ -45,7 +45,7 @@ class ListFragment : Fragment(), OpenDetailsListener {
         recyclerView.adapter = moviesAdapter
         recyclerView.layoutManager = LinearLayoutManager(context)
 
-        viewModel.movies.observe(this, Observer<PagedList<Movie>> {
+        viewModel.movies.observe(viewLifecycleOwner, Observer<PagedList<Movie>> {
             moviesAdapter.submitList(it)
         })
     }