Target Ubuntu in Appveyor CI build

AppVeyor has released support for targetting Ubuntu 18.04 environments.
This adds Ubuntu to the list of build images for Appveyor to use.

appveyor.yml

@@ -2,7 +2,9 @@ version: build-{build}
 branches:
   only:
   - dev
-image: Visual Studio 2017
+image:
+- Visual Studio 2017
+- Ubuntu
 
 nuget:
   disable_publish_on_pr: true
@@ -58,4 +60,4 @@ deploy:
     secure: D+vW2O2LBf/iJb4f+q8fkyIW2VdIYIGxSYLWNrOD4BHlDBZQlJipDbNarWjUr2Kn
   symbol_server: https://www.myget.org/F/rogueexception/symbols/api/v2/package
   on:
-    branch: dev
+    branch: dev