Send build notifications to Slack and remove unused nuget feed

appveyor.yml

@@ -24,3 +24,8 @@ skip_tags: true
 artifacts:
 - path: build\Release\**\*.nupkg
   name: nuget
+
+notifications:
+- provider: Slack
+  incoming_webhook:
+    secure: p5xuu+4x2JHfwGDMDe5KcG1k7gZxqYc4jWVwvyNZv5cvkubPD2waJs5yXMAXZNN7Z63/3PWHb7q4KoY/99AjauYa1nZ4c5qYqRPFRBKTHfA=

nuget.config

@@ -2,7 +2,6 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="wixbuildtools" value="https://ci.appveyor.com/nuget/wixbuildtools" />
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
   </packageSources>
 </configuration>