Updated appveyor file

Author: johelvisguzman

appveyor.yml

@@ -24,6 +24,10 @@ branches:
 #---------------------------------#
 cache:
   - packages
+  
+# scripts that run after cloning repository
+install:
+  - cmd: choco install dotnetcore-sdk --pre
 
 #---------------------------------#
 #       build configuration       #
@@ -35,7 +39,7 @@ platform: Any CPU
 # build Configuration, i.e. Debug, Release, etc.
 configuration: Release
 
-os: Visual Studio 2017
+image: Visual Studio 2017
 
 before_build:
   - dotnet --info