It is available in Debian unstable here, so should be available in Linux Mint eventually, if it is not already.
apt-cache policy synapse
50 tor+http://httpredir.debian.org/debian/ unstable/main amd64 Packages
If it not currently available in Linux Mint Debian Edition, it should be easy to backport if you want to do so.
The build dependencies are:
Build-Depends: debhelper (>= 9),
which are satisfied even on jessie. I was able to build and run it on jessie with no trouble.