Update CI for .NET Core 3.1

ChrisMaddock · ChrisMaddock · commit 3f62216929de · 2020-06-07T16:33:51.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
 language: csharp
 sudo: required
 
-dotnet: 2.1.401
+dotnet: 3.1.300
 mono: 6.6.0
 
 matrix:
diff --git a/appveyor.yml b/appveyor.yml
@@ -2,7 +2,7 @@ branches:
   except:
     - /travis-.*/
 
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 build_script:
   - ps: .\build.ps1 -Target "Appveyor" -Configuration "Release"
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
@@ -7,7 +7,7 @@ jobs:
 
 - job: Windows
   pool:
-    vmImage: vs2017-win2016
+    vmImage: windows-latest
   steps:
 
   - powershell: .\build.ps1 -Target Test -Configuration Release
