Add support for Visual Studio 15 Preview 4. We'll use the Visual Studio 2015 platform toolset for now, this will just allow us to use the IDE without it complaining.

Build/Common.Build.Default.props

@@ -12,6 +12,7 @@
     <PlatformToolset Condition="'$(VisualStudioVersion)'=='11.0'">v110</PlatformToolset>
     <PlatformToolset Condition="'$(VisualStudioVersion)'=='12.0'">v120</PlatformToolset>
     <PlatformToolset Condition="'$(VisualStudioVersion)'=='14.0'">v140</PlatformToolset>
+    <PlatformToolset Condition="'$(VisualStudioVersion)'=='15.0'">v140</PlatformToolset> <!-- v140 is what currently ships in VS15 Preview. -->
   </PropertyGroup>
 
   <!-- Default ChakraDevConfigDir -->