you can actually still install v6. after running the gazebo7_install.sh, run
apt-get remove gazebo7 libgazebo7-dev
apt-get install libsdformat3
apt-get install gazebo6 libgazebo6-dev gazebo6-common
or just use this script instead:
http://osrf-distributions.s3.amazona...bo6_install.sh
at least, that's what's in my notes from last may