Add reference to System.Threading.Tasks when running under .NET 4.0

Author: srijken

Source/Tests/Exceptionless.Tests.csproj

@@ -135,6 +135,9 @@
       <HintPath>..\..\packages\xunit.extensibility.execution.2.1.0\lib\net45\xunit.execution.desktop.dll</HintPath>
       <Private>True</Private>
     </Reference>
+	<Reference Include="System.Threading.Tasks, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" Condition="'$(TargetFrameworkVersionProperty)' == 'NET40'">
+		<Visible>false</Visible>
+	</Reference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="..\GlobalAssemblyInfo.cs">