找了好多，这个是最好的方案了：
转自：http://brightstardb.com/2015/02/24/mono-on-ubuntu.html
# First make sure your base installation is up-to-date
sudo apt-get update
sudo apt-get upgrade

# Install build-essential (may well already be installed, but worth checking)
sudo apt-get install build-essential

# You may want to run the remainder of these commands in a scratch directory

# This grabs the current version of mono (3.12.0)  
# Point your browser to http://download.mono-project.com/sources/mono
# to check that it is still the most recent version.
wget http://download.mono-project.com/sources/mono/mono-3.12.0.tar.bz2
tar -xvf mono-3.12.0.tar.bz2

cd mono-3.12.0
./configure --prefix=/usr/local
make
sudo make install

# This should show that you now have the latest version of Mono installed successfully!
mono --version