Update ReferenceAssembyPath detection

Don't throw exception if `v4.0` path available and `v4.5` is not, plus
check for `v4.5.1`.

Author: gavinfaux

src/ServiceStack.Text/Env.cs

@@ -80,10 +80,12 @@ public static string ReferenceAssembyPath
                 {
                     var programFilesPath = PclExport.Instance.GetEnvironmentVariable("ProgramFiles(x86)") ?? @"C:\Program Files (x86)";
                     var netFxReferenceBasePath = programFilesPath + @"\Reference Assemblies\Microsoft\Framework\.NETFramework\";
-                    if ((netFxReferenceBasePath + @"v4.0\").DirectoryExists())
-                        referenceAssembyPath = netFxReferenceBasePath + @"v4.0\";
-                    if ((netFxReferenceBasePath + @"v4.5\").DirectoryExists())
+                    if ((netFxReferenceBasePath + @"v4.5.1\").DirectoryExists())
+                        referenceAssembyPath = netFxReferenceBasePath + @"v4.5.1\";
+                    else if ((netFxReferenceBasePath + @"v4.5\").DirectoryExists())
                         referenceAssembyPath = netFxReferenceBasePath + @"v4.5\";
+                    else if ((netFxReferenceBasePath + @"v4.0\").DirectoryExists())
+                        referenceAssembyPath = netFxReferenceBasePath + @"v4.0\";
                     else
                         throw new FileNotFoundException(
                             "Could not infer .NET Reference Assemblies path, e.g '{0}'.\n".Fmt(netFxReferenceBasePath + @"v4.0\") +