Explicitly state protocol v3

Author: AArnott

src/nuget.config

@@ -4,8 +4,9 @@
     <add key="repositorypath" value="packages" />
   </config>
   <packageSources>
-    <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="myget.org/F/aarnott" value="https://www.myget.org/F/aarnott/api/v3/index.json" />
+    <clear />
+    <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+    <add key="myget.org/F/aarnott" value="https://www.myget.org/F/aarnott/api/v3/index.json" protocolVersion="3" />
     <add key="PInvoke CI" value="https://ci.appveyor.com/nuget/pinvoke" />
     <add key="AppVeyor CI" value="https://ci.appveyor.com/nuget/nerdbank-gitversioning" />
   </packageSources>