enable net451 framework again

Author: dotarj

src/PartialResponse.AspNetCore.Mvc.Formatters.Json/project.json

@@ -26,6 +26,7 @@
     "Newtonsoft.Json": "9.0.1"
   },
   "frameworks": {
+    "net451": {},
     "netstandard1.6": {}
   }
 }