Try explicitly set FX version

Author: hazzik

doc/NHibernate.shfbproj

@@ -28,6 +28,7 @@
       <DocumentationSource sourceFile="../src/NHibernate/bin/$(Configuration)/net461/NHibernate.dll" />
       <DocumentationSource sourceFile="../src/NHibernate/bin/$(Configuration)/net461/Nhibernate.xml" />
     </DocumentationSources>
+    <FrameworkVersion>.NET Framework 4.6.1</FrameworkVersion>
   </PropertyGroup>
   <!-- There are no properties for these two groups but they need to appear in
        order for Visual Studio to perform the build. -->