Set dnx to 461 as there's no real 51

Ix.NET/Source/Tests/project.json

@@ -42,7 +42,7 @@
                 "System.Threading.Tasks": "4.0.0.0"
             }
         },
-    "dnx51": {
+    "dnx461": {
       "compilationOptions": {
         "define": [
             "NO_EVENTARGS_CONSTRAINT",