deactivate 18.04

because the dockerfile does not build anymore:

92.11 Errors were encountered while processing:
92.11  /tmp/apt-dpkg-install-D2nJme/660-robotpkg-openscenegraph_3.6.5r3_amd64.deb
92.11  /tmp/apt-dpkg-install-D2nJme/661-robotpkg-py36-eigenpy_3.3.0_amd64.deb
92.11  /tmp/apt-dpkg-install-D2nJme/664-robotpkg-qt5-osgqt_3.5.7r3_amd64.deb