Updating ubuntu linux
The oracle-java8-installer package now sets the Java priority to 1081, and that may or may not set it as default, depending on other Java packages you may have installed (for instance, if Java 7 is also installed, Java 8 becomes default, but if Java 9 is installed, Java 8 doesn't become default).
Login to the box running the old target release of Ubuntu (12.04 Server LTS in my case).
If for some reason you want to pin an older version of Mono rather than updating to the latest, you can modify the Debian repository to “wheezy/snapshots/X. These snapshots will cease receiving updates as soon as the next major Mono version gets uploaded - for example, as soon as Mono 3.12 gets uploaded, 3.10 will never receive updates.