As Ubuntu is based on Debian, there is a simple and hacky workaround:
> mkdir debs && cd debs
> wget http://fr.archive.ubuntu.com/ubuntu/pool/main/g/gst-plugins-base0.10/libgstreamer-plugins-base0.10-0_0.10.36-1_amd64.deb
> wget wget http://fr.archive.ubuntu.com/ubuntu/pool/universe/g/gstreamer0.10/libgstreamer0.10-0_0.10.36-1.5ubuntu1_amd64.deb
> sudo dpkg --install *
> uname -a && lsb_release -a && /opt/PixInsight/bin/PixInsight.sh
Linux debian-stretch-testing 4.7.0-1-amd64 #1 SMP Debian 4.7.8-1 (2016-10-19) x86_64 GNU/Linux
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux testing (stretch)
Release: testing
Codename: stretch
PixInsight Core 01.08.04.1198 Ripley (x64)
Copyright (c) 2003-2016 Pleiades Astrophoto
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Cheers
Thomas