ci: install .NET

Author: felixfbecker

.travis.yml

@@ -3,6 +3,8 @@ language: minimal
 dist: bionic
 
 before_install:
+  # Install .NET
+  - curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin
   # Download the Microsoft repository GPG keys
   - wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb
   # Register the Microsoft repository GPG keys