Use .NET Core 3.1 on Travis

Author: oschwald

.travis.yml

@@ -11,12 +11,12 @@ env:
 matrix:
   include:
   - os: linux
-    dotnet: 3.0
+    dotnet: 3.1
     mono: none
     sudo: required
     env: DOTNETCORE=1 CONSOLE_FRAMEWORK=netcoreapp3.0
   - os: osx
-    dotnet: 3.0.101
+    dotnet: 3.1.100
     mono: none
     env: DOTNETCORE=1 CONSOLE_FRAMEWORK=netcoreapp3.0
   - os: linux