Added reference to "System.Net.Http.Formatting"

AppVeyor failed to build the project without it.

Author: leekelleher

src/Our.Umbraco.NestedContent/Our.Umbraco.NestedContent.csproj

@@ -59,6 +59,7 @@
     <Reference Include="System" />
     <Reference Include="System.ComponentModel.DataAnnotations" />
     <Reference Include="System.Core" />
+    <Reference Include="System.Net.Http.Formatting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL" />
     <Reference Include="System.Web" />
     <Reference Include="System.Web.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
       <HintPath>..\packages\Microsoft.AspNet.WebApi.Core.4.0.30506.0\lib\net40\System.Web.Http.dll</HintPath>