Filter out symbols source packages

Since these pushes fail every time, skip them for now and take care
of later.

Ref: #6.

appveyor.yml

@@ -52,7 +52,7 @@ deploy_script:
           return;
       }
       mkdir dist
-      $artifacts.keys | % { $artifacts[$_]['sourcePath'] } | Copy-Item -Destination dist
+      $artifacts.keys | % { $artifacts[$_]['sourcePath'] } | ? { $_ -notlike '*.symbols.nupkg' } | Copy-Item -Destination dist
       $packages = ls dist | select -ExpandProperty FullName | ? { $_ -notlike '*.symbols.nupkg' }
       $packages | % { dotnet nuget push $_ -k $env:NUGET_API_KEY -sk $env:NUGET_API_KEY -s https://nuget.org/api/v2/package -ss https://nuget.smbsrc.net/ }
       if ($LastExitCode -ne 0) { throw; }