$apt-get install subversion
Reading package lists… Done
Building dependency tree
Reading state information… Done
Package subversion is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or is only available from another source
Go to System > Administration > Software Sources
Check all options (main, universe, restricted and multiverse – actually only main is required but in case you run into more problems, check all), close
Now run apt-get install subversion, it should work!!