Integration tests use different refs for `base-platform` and `base-sdk`

This has the unfortunate side effect of downloading multiple versions of the largest packages in the project, which probably causes a 10-20 minute overhead.

Quoting @tristanvb from an IRC conversation on this topic:

<tristan> What should be done: A.) Use the new ref all around, B.) dont stage *both* gnome-platform and gnome-sdk, only the sdk with the proper symlinks trick
<tristan> which can be found in gnome-modulesets-base repo, there is the correct way to do it
<tristan> Also, possibly updating the base causes binary output to be different, in which case, integration tests should be refactored as mentioned in a bug report I filed

The mentioned issue is #126 (closed).