Install aptitude before trying to use it.

.travis.yml

@@ -4,6 +4,7 @@ install:
   - 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
   - sudo apt-get update -qq
+  - sudo apt-get install -y aptitude
   - sudo aptitude install mono-devel nuget
   - build/premake5-linux-64 --file=build/premake4.lua gmake
   - nuget install NUnit.Runners -Version 2.6.4 -OutputDirectory testrunner