Limit NHibernate dependency version to 5.1.x to avoid breaking changes in 5.2.x (see #104)

Author: peetw

NHibernate.Spatial/NHibernate.Spatial.csproj

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="NetTopologySuite" Version="1.15.1" />
-    <PackageReference Include="NHibernate" Version="5.1.3" />
+    <PackageReference Include="NHibernate" Version="[5.1, 5.2)" />
   </ItemGroup>
 
 </Project>