Lock Nuget NHibernate versions to < 4.0.

The current FluentNHibernate version on Nuget depens on NH >= 3.3.1.4000. I believe it would be helpful to tighten this dependency to >= 3.3.1.4000 and < 4.0, in preparation for NH 4.0 release.

The NH 4.0 release could happen any day now.