Last try for Travis to see if Artemis build against Mono at all

.travis.yml

@@ -1,2 +1,4 @@
 language: csharp
+mono:
+  - latest
 solution: ./src/Artemis.sln

src/NuGet.Config

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <add key="nuget.org" value="https://www.nuget.org/api/v2" />
-    <add key="RGB.NET MyGet" value="https://www.myget.org/F/rgb_net/api/v2" />
-  </packageSources>
+  <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+  <add key="RGB.NET MyGet" value="https://www.myget.org/F/rgb_net/api/v3/index.json" />
+</packageSources>
 </configuration>