Update oppia xblock to point back upstream.
Update oppia xblock to point back upstream.

Oppia has merged our PR to allow them to run on python 3 so we're updating
the pointer to the merge commit upstream.

EDUlib/configuration,mitodl/configuration,stvstnfrd/configuration,mitodl/configuration,EDUlib/configuration,edx/configuration,stvstnfrd/configuration,mitodl/configuration,arbrandes/edx-configuration,arbrandes/edx-configuration,EDUlib/configuration,edx/configuration,mitodl/configuration,stvstnfrd/configuration,stvstnfrd/configuration,stvstnfrd/configuration,EDUlib/configuration,arbrandes/edx-configuration,edx/configuration,edx/configuration,EDUlib/configuration,arbrandes/edx-configuration,arbrandes/edx-configuration