At the time of writing 3.8.0 was the newest version for Mono that I could find. If a newer version of Mono has come out by now, you can probably find it here.

Installation guide

Make might take 30-60 minutes, make-install should be fast

apt-get update
apt-get install -y --force-yes gcc libtool bison pkg-config libglib2.0-dev gettext make bzip2 g++ build-essential

mkdir -p /var/mono-install
cd /var/mono-install

wget http://origin-download.mono-project.com/sources/mono/mono-3.8.0.tar.bz2
tar xvjf mono-3.*.tar.bz2
cd mono-3.*

./configure --prefix=/opt/mono3
make
make install

rm /usr/bin/mono
rm /usr/bin/gmcs
rm /usr/bin/mcs
rm /usr/bin/smcs
rm /usr/bin/dmcs
ln -s /opt/mono3/bin/mono /usr/bin/mono
ln -s /opt/mono3/bin/gmcs /usr/bin/gmcs
ln -s /opt/mono3/bin/mcs /usr/bin/mcs
ln -s /opt/mono3/bin/smcs /usr/bin/smcs
ln -s /opt/mono3/bin/dmcs /usr/bin/dmcs

rm -R /var/mono-install