Move to CoreFX build rc2-23901

Update .exe's project.json Target Framework from dnxcore50 to netstandardapp1.5.
Update .dll's project.json Target Framework from dnxcore50 to netstandard1.3.

Adding workaround for DataContractSerialization to src\dotnet\project.json to fix crossgen issue.
Build 23901 has a dependency issue that doesn't allow the runtime.any.System.Private.DataContractSerialization
package to be restored. When we move to a new build of CoreFX we should take this workaround out.

src/dotnet/Program.cs

@@ -123,7 +123,7 @@ namespace Microsoft.DotNet.Cli
                 return builtIn(appArgs.ToArray());
             }
 
-            return Command.Create("dotnet-" + command, appArgs, FrameworkConstants.CommonFrameworks.DnxCore50)
+            return Command.Create("dotnet-" + command, appArgs, FrameworkConstants.CommonFrameworks.NetStandardApp15)
                 .ForwardStdErr()
                 .ForwardStdOut()
                 .Execute()