Update appveyor

to include linux

Author: Mpdreamz

appveyor.yml

@@ -1,5 +1,7 @@
 version: 1.0.{build} 
-image: Visual Studio 2017 
+image:
+- Visual Studio 2017
+- Ubuntu
 test: off
 environment:
     DOTNET_CLI_TELEMETRY_OPTOUT: true
@@ -18,3 +20,11 @@ for:
   artifacts:
       - path: .\build\output\_packages\*.nupkg
         name: NuGet
+
+-
+  matrix:
+    only:
+      - image: Ubuntu
+
+  build_script:
+  - ./build.sh test-one skipdocs