Target net451 rather than dnx451

Class libraries should target net451 rather than dnx451, according to this: aspnet/Announcements#98

Also updated global.json to use DNX v1.0.0-rc1-update1

Dapper.StrongName/project.json

@@ -33,7 +33,7 @@
         "System.Data": "4.0.0.0"
       }
     },
-    "dnx451": {
+    "net451": {
       "frameworkAssemblies": {
         "System.Data": "4.0.0.0",
         "System.Xml": "4.0.0.0"

Dapper/project.json

@@ -31,7 +31,7 @@
         "System.Xml": "4.0.0.0"
       }
     },
-    "dnx451": {
+    "net451": {
       "frameworkAssemblies": {
         "System.Data": "4.0.0.0",
         "System.Xml": "4.0.0.0"

global.json

 {
   "sdk": {
-    "version": "1.0.0-rc1-final"
+    "version": "1.0.0-rc1-update1"
   },
   "projects": [
       "Dapper", "Dapper.EntityFramework", "Dapper.Contrib", "Dapper.Rainbow", "Dapper.SqlBuilder", "Dapper.Tests"