Update Travis .NET Core SDK version

bording · bording · commit 9a67257a8879 · 2019-04-20T13:09:13.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -3,7 +3,7 @@
 
 language: csharp
 dist: trusty
-dotnet: 2.1.401
+dotnet: 2.1.506
 mono: none
 osx_image: xcode8.3
 
