Target .NET 4.5.1 as well as .NET Standard 1.6

It is possible to build ASP.NET Core applications that target the .NET
4.5 framework. Because this package only targets .NET Standard 1.6 it
cannot be imported into ASP.NET Core projects that do so. This is a very
simple change and is risk free, while broadening the appeal of solution.

Author: JSkimming

src/Stormpath.AspNetCore/project.json

@@ -21,6 +21,7 @@
   },
   "description": "Stormpath middleware for ASP.NET Core. Easily adds authentication and authorization to ASP.NET Core applications.",
   "frameworks": {
+    "net451": {},
     "netstandard1.6": {
       "imports": "dotnet"
     }