Update dotnet install to use ps1 script

Signed-off-by: Darren Stahl <darst@microsoft.com>

Author: darstahl

appveyor.yml

@@ -2,8 +2,8 @@ version: '{build}'
 pull_requests:
   do_not_increment_build_number: true
 init:
-- ps: (new-object net.webclient).DownloadFile('https://dotnetcli.blob.core.windows.net/dotnet/beta/Installers/Latest/dotnet-dev-win-x64.latest.exe', "c:/dotnet-install.exe")
-- ps: Start-Process c:\dotnet-install.exe -ArgumentList "/install","/quiet" -Wait
+- ps: (new-object net.webclient).DownloadFile('https://dot.net/v1/dotnet-install.ps1', "C:/dotnet-install.ps1")
+- ps: C:/dotnet-install.ps1 -Channel 2.0
 nuget:
   disable_publish_on_pr: true
 build_script: