Install .NET Core SDK 3.1

Author: JoeRobich

.travis.yml

@@ -17,7 +17,7 @@ before_install:
   - sudo dpkg -i packages-microsoft-prod.deb
   - sudo apt-get install apt-transport-https
   - sudo apt-get update
-  - sudo apt-get install dotnet-sdk-2.2
+  - sudo apt-get install dotnet-sdk-3.1
   - sudo apt install xvfb
 
 addons: