Pin Linux CI builds to Mono 3.12.0

Author: nulltoken

CI/travis.linux.install.deps.sh

@@ -3,7 +3,7 @@ set -ev
 
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
 
-echo "deb http://download.mono-project.com/repo/debian wheezy main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
+echo "deb http://download.mono-project.com/repo/debian wheezy/snapshots/3.12.0 main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
 echo "deb http://download.mono-project.com/repo/debian wheezy-libtiff-compat main" | sudo tee -a /etc/apt/sources.list.d/mono-xamarin.list
 
 sudo apt-get update