Rollback to mono 4.2.4.4 (pythonnet#233)

Experiencing bug in travis build similar to
https://bugzilla.xamarin.com/show_bug.cgi?id=42058

.travis.yml

@@ -10,7 +10,7 @@ before_install:
   - sudo add-apt-repository -y "deb http://archive.ubuntu.com/ubuntu/ trusty main universe"
   - sudo apt-get install software-properties-common
   - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --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/4.2.4.4 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
   - sudo DEBIAN_FRONTEND=noninteractive apt-get -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confnew" install mono-devel mono-complete referenceassemblies-pcl ca-certificates-mono nunit-console