Allow Downgrade option

[robinpecha]

We are using this method to deploy Docker to our servers. It would be very helpful to have a switch that allows downgrading, in case Docker is accidentally upgraded to a higher version on some servers. Currently, we are modifying your script with the following command to enable downgrades:
sed -i 's/apt-get -y -qq install/apt-get -y --allow-downgrades -qq install/g' get-docker.sh # enables downgrade

[LuckyWhel]

sed -i 's/apt-get -y -qq install/apt-get -y --allow-downgrades -qq install/g' get-docker.sh # enables downgrade